A Spider and an Undead, So What?

A Spider and an Undead, So What? (蜘蛛と不死者ですが、なにか？) is a collaboration game event in Mass for the Dead with characters from Overlord and So I'm a Spider, So What?. It was released on January 14, 2022 to February 15, 2022. The event was re-released on February 15, 2023 to March 1, 2023.

Background
Regular summoning experiment in Nazarick. The guests from another world summoned this time are a girl who claims to be the Demon King and a spider. Momonga tries to negotiate a ceasefire while being puzzled by the first case when hostiles are summoned to Nazarick at the same time.

Incident
In another world, the armies of Sariella and Ohts war at the Zatona Plains. However immediately after the start of the battle, the battlefield appears to have exceeded the expectations of both sides due to the intervention of the monster known as the "Nightmare of the Labyrinth" or Kumoko that launched an attack on Ohts. And the situation was made even more chaotic with another mighty existence appeared on the battlefield, Ariel the Demon Lord. Kumoko fights a defensive battle against the vindictive Demon Lord seeking to destroy the spider monster, barely staying alive with through absorbing the experience points by consuming the nearby soldiers in the vicinity. Ariel aware that in order to defeat her foe quickly prepares to use Abyss Magic, but before she does so, a Crack forms behind her sucking her into it. Kumoko receiving a reprieve from being destroyed by Ariel soon becomes horrified that she too is pulled into the anomaly. A boy with a blue muffler who had been standing just a few feet away is left shocked by the monsters' disappearances.

In the Great Tomb of Nazarick on the 10th Floor, Momonga and Demiurge prepare for another summoning experiment using the Crack at the Throne Room. As preparations are being made, Momonga and Demiurge speak, with the Overlord thanking Demiurge for his invaluable services in the experiments. The two understand that this summoning will no doubt bring some new guests from another world who seem to be exceptions for the Crack phenomena not only in their abilities but also the information they possess. Strangely as far as the two can tell on the nature of the Crack, the visitors always return to their original world after a certain period of time. In any given it seems that Cracks do form in other worlds because they are drawn to existences with a certain amount of "power" and try to pull them in. Though it could also mean that the Cracks form on other worlds with multiple gatherings of people given what they know about Chaos Beasts, but so far the other worlds have yet to encounter Chaos Beasts. This makes Momonga wonder if perhaps the Cracks formed in other worlds and the Chaos Beasts are truly a result of the same phenomenon.

There are still too many unknowns and Demiurge hopes that the new summoning will bring forth some interesting results. Momonga is more concerned on whether their upcoming guests will be cooperative. Demiurge believes that they should be as in past summoning Nazarick has been overall successful in building friendly relationships with visitors from other worlds. However Momonga believes that such relationships could just be consequential of the Cracks, surmising that being summoned from them causes a mental effect that makes them more friendly to outsiders. Demiurge sees his master's point and agrees that he will take extra care when interacting with the summons. Momonga muses that it would be a good thing perhaps if their guests could permanently become part of Nazarick's forces. Though Momonga mentally notes that such beings normally would want to return back to their original worlds and any attempt to interfere would lead to hostility. The experiment begins with Momonga's permission and the undead guards stand ready to receive their guests.

Soon enough the Crack opens and Ariel and Kumoko are spat out. Kumoko regaining her bearing sees that she and her foe are in the midst of many undead monsters. The two postpone their battle to get a handle of the situation and direct their attention to Momonga. Kumoko tries to use [https://kumodesu.fandom.com/wiki/System/Skills/Appraisal#:~:text=Appraisal%20is%20the%20first%20skill,this%20skill%20to%20level%2010. Appraisal] on the Overlord only to have the Divine Voice inform her that the skill was interrupted, causing the spider to realize that the being before them was powerful and on par with an Administrator.

Equally Momonga views his new guests with curiosity as well as caution. He is unable discern anything with his spells of [Life Essence] or [Mana Essence], but notes that there seems to be some sort of tenuous relationship between the spider and girl. Momonga makes his introduction, assuring his visitors that he means no hostility. His mannerism perturb Kumoko finding the skeleton's attitude to be similar to her countryman from her original world. After hearing that Momonga wishes to have a peaceful exchange of information, Ariel is a bit annoyed that her business between her and Kumoko was interfered with by Momonga's summoning. Demiurge takes charge of the conversation, offering a full apology for the inconvenience, admitting that they cannot deny the possibility that Nazarick was the cause of the summoning, but also adds they may have rescued the two from great danger. The last reason piques Ariel's interest and the 7th Floor Guardian goes on to offer further details once they are assured no further hostilities will be committed. Ariel finds this to be acceptable, as does Kumoko who allow the two to elaborate on the current situation.

An exchange of information is give, with Kumoko, being unable to convey her thoughts to Nazarick's representative, listens to the ambiguity that the summoning may or may not be triggered by a Crack in another world or if the summoning experiment actually rescued her and Ariel from doom. Ariel is unable to determine which is the case, though makes it clear that it would make Momonga either a hindrance or benefactor to her. For now, the Demon Lord will not retaliate against nor thank the Overlord. From what Ariel can gather she and her associate will be granted sanctuary in Nazarick in return for some requested work, but essentials such as food, clothing and shelter. Additionally Momonga also requests information of their world, though states that their movements outside of Nazarick will be limited. Ariel assumes that the information Momonga seeks includes both her and Kumoko's fighting styles and strengths, which Momonga confirms. Ariel is still wary of accepting the protection of a being such as Momonga, and still wishes to settle her score against Kumoko, but is aware that she cannot take hostile action against Momonga and Demiurge knowing they are powerful and too risky to anger. Not only that her mental condition shows that her soul and personality and beginning to merge with Kumoko's Parallel Mind. If she kills the spider there still is no guarantee that the merging will halt. So Ariel turns her focus on finding a way to return to her world and enjoying the comforts offered. Momonga after getting an approval from Ariel, turns to Kumoko, who unfortunately though understanding Momonga's deal is not able to verbally converse unless via Telepathy. Momonga seeing that the communication barrier to be an issue tries to use [Message] to communicate finds that it is useless, thus forcing Kumoko to convey her responses in the affirmative or negative via raising her right or left claw.

The visitors to Nazarick are given separate Guest Rooms. Kumoko in her own room ruminates over he situation and to test out if her skills have been affected with the lack of the System. So far she confirms that he magic is working properly, though considers testing out her teleportation magic. Though she remembers that Momonga warned her that teleport magic is not so reliable in the current world due to the Cracks. While it could be just a lie to keep her contained in Nazarick, Kumoko does not want to risk using that magic as she can't specify specific coordinates that well. Even though the spider wishes to remain and live her life freely, staying in Nazarick is her best option to protect herself from the Demon Lord that is trying to destroy her. Kumoko believes her best option to to build up her experience and evolve quickly into an Arachne in order to match Ariel and also find a countermeasure against Abyss Magic. Kumoko's Evil Eyes suddenly activate and cause a explosion on the floor where a monster was hiding in a shadow. Through her Wisdom, Kumoko determines that it was a Shadow Demon and placed in her room to observe her at Momonga's orders. The spider wonders if she may have just committed a unwise act, when Momonga visits her room to investigate the explosion he detected. Momonga makes an apology to Kumoko for the surveillance and gives his assurances that the spider is indeed safe in Nazarick. Despite the reassurances from Momonga, Kumoko still had doubts about her future treatment while under his protection.

Immediately after Ariel and Kumoko were welcomed into the tomb, Demiurge convened a meeting of the Floor Guardians in the Round Table Room. The topic of discussion, is to make their guests take permanent residence in Nazarick. The idea is a bit odd to Albedo and Aura who think that Demiurge suggests this because their latest guests are exceptional than normal. This is not the case, as Demiurge states that it is still too early to tell. In any case this idea is from his divining of Momonga's will from his conversation he had with him before the start of the summoning. Others are a bit skeptical that visitors from others world can even live permanently in their world. Sebas Tian present at the meeting had always assumed that the returning was part of built in mechanism of the summoning and that there was a limit of how long visitors could stay. The speculation is not doubted by Demiurge but he believes that the visitors themselves actually activate the return to their world if their desire is strong enough. Instead he proposes that they test out if visitors from other worlds can have a fixed existence in their world. To do this the Guardians most not do if forcibly but make their guests feel accepted and they ensure they are comfortable in Nazarick such much that they do not wish to return to their original world anymore. Demiurge understand his colleagues reluctance to proceed, as this move is a bit radical, as they had only trusted other creations of the Supreme Beings or their summons and always shunned outsiders. Sebas Tian speaks in favor of the plan, noting that there were precedents where they accepted outsiders into Nazarick as companions and contributors such as Slimeko and Kyuko. The others soon agree to the proposal, as they believe that they are working in Momonga's interest and see it as a test to their loyalty to him to ensure his wishes are fulfilled by providing to be as hospitable to their guests as possible that they voluntarily stay as members of Nazarick.

Later on the 6th Floor, Kumoko and Ariel wait to begin their first job. Kumoko is nervous, more so being in the presence of the Demon Lord who tried to kill her than the upcoming task. The Demon Lord notices the spider's tension, though oblivious to her fear, thinking that Kumoko eat something bad. The change in attitude causes Kumoko to be ever cautious though considers that Ariel's mind is being influenced by her Parallel Mind. The change is not missed by Ariel who reflects on her shift in personality. Yuri Alpha who is acting as their chaperone ask Ariel if anything is wrong, only to be told by the girl that she was wondering if she should use Telepathy to better communicate with the spider, but advises against it since a user needs some delicate skill. Aura Bella Fiora and Mare Bello Fiore finally arrive at the scene to guide their new guests through the task that they have been requested to do. Ariel upon seeing the twins are elves becomes aggressive, stunning the two Floor Guardians on her hostility. The Demon Lord though noticing her unsightly demeanor quickly calms down and apologies though asks if they ever heard the name Potimas. The name is not familiar to neither Aura or Mare. Both twins think Ariel's aggression against them is that fact that they belong to the elven race though make it clear they consider themselves to be distinct from elves as they are dark elves. While Ariel apologies for her behavior to the twins, Kumoko notices that the two Floor Guardians were prepared to fight the Demon Lord as if it was natural, telling the spider that the elves were not normal. Aura and Mare then bring their guests to the nearby Crack where the Chaos Beast subjugation can begin.

At the site of the Crack, Ariel and Kumoko make short work of the Chaos Beasts. Sadly Kumoko finds that despite consuming Chaos Beasts they do not provide any experience points for her to gain. Once the subjugation is done, Yuri and the dark elves thank the Demon Lord and spider for their assistance. After their guests are guided back to their rooms on the 9th Floor, Yuri, Aura and Mare report to the Round Table Room to give their impressions of the visitors to Momonga and Albedo. From what Yuri can tell of Ariel, she appears to be a little old-fashion in her attacks, preferring to maintain her distance to minimize danger. For Kumoko, Aura finds the spider's fighting style to be similar to Momonga which intrigues the undead. The bottom line from Albedo is that the two possess unique fighting abilities and are seasoned fighters which to Momonga means that he cannot afford to underestimate them. He decides to follow his comrade Punitto Moe's words to emphasize on more information gathering for his new guests. Momonga is also informed of the prejudice Ariel has for elves, which he assures Aura and Mare to not be troubled over it. In fact he blames himself for the incident as he always sent the twins to engage with visitors from the Crack under the presumption that the visitors from other worlds would be human and respond better to beings similar to them. In this situation that is not the case which he apologies for them having experience such brief hostility. The matter at hand now is that Momonga still does not know Ariel and Kumoko's full abilities, though he believes the could rival him in power. Albedo keeps his worries down, stating that she and the other will be as cooperative with the two guests for him. The words startle Momonga as he did not expect such encouragement from Albedo, especially with outsiders but thinks that this is most efficient. Little does he know the Guardians are scheming to increase friendly contact with their guests to add their strength to Nazarick.

A few days after Ariel and Kumoko were summoned to Nazarick, Kumoko finds that the treatment at the tomb has been good, enjoying the luxuries such as the food, soft beds, baths and having a drink at the bar. While out on a mission outside Nazarick, Kumoko bonds with Entoma Vasilissa Zeta who is acting as a guide. Both Kumoko and maid get along over due to being both spiders and also for their preferences for specific "food". Still neither Ariel or Kumoko are allowed to go out by themselves unescorted. The fact that Entoma and Cocytus are both present with her, and are also insect types dawns on Kumoko that they are especially being nice to her. She finds the company to be not disagreeable given her isolation spent in the crazy labyrinth even if her new associates are subtly extracting information from her. On this currenting outing with Kumoko, it is to get her some prey to feed on to do a test trial to see if Kumoko can gain any experience points though have the maid and Floor Guardian presence are explained to be for her protection since monsters like them are deemed threats by the humans. Entoma's insect swarm locates a nest of nearby demon beasts and the trio soon arrive at a cave where Kumoko can feed. Not only does Kumoko feast but Entoma as well on the monsters. Kumoko after eating her prey finds that the demon beasts she consumes do indeed accrue as experience points. Though compared to human flesh the meat is not very tasty. Kumoko notices that her hosts do not have an issue on her feasting on humans, and their ethics make them very flexible when killing humans such as criminals out of practicality. For that matter Kumoko finds that Momonga's policy of "hostility for hostility, respect for respect" and "giving grace to grace" in line with her morals. Kumoko even dares think that despite the monsters inhabiting Nazarick, it could be actually a heaven to her. However Kumoko still is a bit crossed over Momonga's part in pulling her into this world against her will.

Another couple of days pass after Kumoko had confirmed that it was possible to gain experience points, and Ariel finds herself in the Amphitheater. The Demon Lord is all too aware that the treatment se received from Nazarick is good and for the most part the inhabitants are eerily friendly, so much so that she sees that Kumoko may not want to go back to the other world. Ariel on the other hand still desires to return but so far o clues of how elude her. Ariel was allowed to try an return method that Nazarick previously used as a precedent in the Great Forest of Tob. It involved attacking the activated Crack to break the stability of space and use summoning to push through the barrier between worlds and open a portal, but the attempt failed either because the gulf between worlds was too great compared to last time or maybe "someone" is interfering. Without any way to go home, Ariel is stuck under the care of Momonga's followers and must cooperate with his requests during her stay. Shalltear Bloodfallen arrives to greet the Demon Lord to start the latest request from Momonga. Ariel correctly guesses that it involves fighting Shalltear in a mock battle. But besides its purpose is to gauge Ariel's abilities, Areil believes the battle is to also assist Shalltear in giving her much needed combat experience against a powerful opponent. Ariel is not unfavorable to the idea as she too wants to know exactly how strong a Floor Guardian is. To prepare for battle, Shalltear wears a collar to help her in gaining experience points. Just before they begin their duel, Ariel realizes that Shalltear seems to belong to a specific "humanoid race".

The two engage in a battle, trading blows with each other as a small audience that includes Kumoko, Entoma Lupusregina and Solution. Solution observe that Ariel's Gluttony skill wipes out magic, putting Shalltear at a disadvantage, but the maid takes into account Shalltear had already gave herself a handicap when she wore a collar and suspects that the Demon Lord is not truly showing how strong she is, causing Solution to dare ask what would the battle be if both were at their best. The battle ends with Ariel knocking off Shalltear off her feet. Ariel is a bit disappointed and suggests that her sparring partner remove her collar to fight her equally, but is turned down by Shalltear who prefers this to be more interesting caring not for her pride and only on her mission to serve Momonga. This acts impresses Ariel as such loyalty is rarely found among her own demons. The Demon Lord asks why the vampire wants to regain her strength, only to be told that she wants to obtain what her creator had given her. The reason seems complicated to Ariel but she accepts it, hoping Shalltear succeeds as she would like to battle again. Kumoko watching the interaction is confused by the slight but subtly friendly banter and cannot help but compare it to a scene from a boy's manga.

Although these is no sign of finding a means to return Kumoko and Ariel have become accustomed to life in Nazarick, and have been brought forth for their fighting ability and receive requests for mock combat. Another fight between the visitors from the other world and Floor Guardians ensues, in the Amphitheater, this time against Cocytus and Shalltear. Ironically Ariel and Kumoko fight themselves fighting together against their opponents. The turn of events is unexpected to Kumoko though suspects that her Parallel Mind has influenced the Demon Lord enough to allow this cooperation and even become friendly with her. Combat ceases when Momonga appears at the scene to see how well the two are doing. Ariel is surprised that the leader of the tomb would grace them with his presence. Momonga having some free time, offers an interesting challenge, the Demon Lord and her spider friend fight against him, the Floor Guardians and Sebas Tian. The change in routine is unexpected to Ariel, as she had assumed that the undead would try to hide the abilities of him and his subordinates as much as possible so that they maintain their advantage over her and the spider. Momonga does not deny that he has been keeping his cards close, but finds that this newest mock battle will be a test to see the depths of each other strengths for all honesty sake. While Ariel is suspicious of Momonga's motives for providing this opportunity, Momonga secretly intends to use this battle to help strength his Guardians in preparation against the unknown entity that destroyed Nazarick. When that entity returns it will be total war and Nazarick needs to be train harder for that day. Ariel as well as Kumoko are opponents that do not refrain from pulling their attacks and so his Guardians will get as much experience from this as possible. Ariel agrees to the request with guarded wariness and Kumoko oblivious to the tensions goes with the flow more so to impress Entoma who is watching at the sidelines.

The battle between Momonga's forces and the visitors take place. Ariel and Kumoko team up to pierce through a defensive wall that surrounds Momonga. It is broken through by Ariel, which allows Kumoko to finally reach Momonga, but is blown back by an [Explosive Land Mine] that Momonga planted. The battle is deemed to be done and Momonga ends it with both parties exhausted. Kumoko is amazed by the whole strategy, where Momonga used his own minions' casual positioning as a trap for his opponents. Momonga though feels he did not fully see Ariel's true strength, not without the use of Super-Tier Magic or a World Item. In regards to Kumoko, he finds that the spider to be as equally as crafty and him, making use of all her assets and was rather surprised that she survived his explosive. On a common note, both seem to be out of depth on Ariel's strength. Despite the lack of progress, Momonga and Kumoko, though rivals in battle, found it to be equally fun.

Aftermath
A while since Kumoko and Ariel came to Nazarick, Momonga meets with Albedo and Demiurge at the Round Table to discuss about their guests. Ariel appears to be dissatisfied with the lack of progress of finding a means to return to her world. Though her dissatisfaction is not so that it has made her refuse request and is generally at peace. For Kumoko things are going quite well. Direct communication is difficult, though according to Ariel, she could connect Kumoko's mind to a recipient when they are drunk. From these conversations, Albedo reports that the spider views Nazarick in a favorable way, comparing it as a "heaven". Additionally the spider has made friends with not just Entoma but also the Eight-Edge Assassins. Kumoko's familiarity with Nazarick has become stronger and Demiurge advises Momonga that he should use this chance to fulfill his wish for a guest from another world to remain as a permanent resident with Kumoko since she is most likely to stay. The idea of a making one of the guests a permanent residence confounds Momonga bit, especially when the two Guardians claim that it was his "wish". It appears that it was the result of a misunderstanding between him and Demiurge at Crack just before they began the summoning. Momonga seeing that the Guardians have been working to this goal under his presumed wishes, is unsure about accepting the outsider, but understands that acquiring strength for Nazarick takes a precedence, though only if Kumoko accepts willingly. He goes along with the charade and orders Kumoko to be brought before him.

The spider finding she was summoned before the leader of Nazarick, panics thinking she may have done something to offend the undead. Momonga though gets straight to the point and offers the spider an invitation to remain permanently in Nazarick finding her familiarity with his subjects and strength to be good qualifiers for her to remain. The offer shocks Kumoko who is told by her host that it does not come lightly as Nazarick has prejudice against outsiders, only rarely accepting Kumoko since she is a heteromorph. Kumoko is unsure of the offer, believing it may be too good to be true. While she knows of Nazarick's hostility towards humans add has no taste for it, she really has no issue. Her concern drifts toward the Demon Lord who may return to the other world. Strangely the spider finds no reason of why she needs to go back. There is some worry over the vampire child in the other world, but is afraid of the danger of returning. Beside she tells herself that the other world is already crowded as it is.

Kumoko is about to pantomime to Momonga to be given some more time to decide, but before she can tell him a voice rings out in Kumoko ear's belong to D. D's smartphone terminal appear and connects Kumoko to the spider. Time having stopped the spider listens to the goddess apologize for the time it took to find the spider claiming she was utterly surprised when she and Ariel vanished. Kumoko does not buy it and correctly guesses that the goddess was aware of what happen for some time as the time of her intervention is too much convenient. The goddess admits that she allowed the events to play out because she found them interesting, however hearing that Kumoko got an offer to remain in Nazarick forced D to intervene as if the spider stays then it would be boring. Kumoko seeing that she has no say in the matter can only accept that D will have her way. Before D can send Kumoko back to the other world, Momonga speaks out to the surprise of the two. Momonga using his immunity against time magic enters the conversation. Knowing that his friend will be leaving is a bit disappointed that Kumoko's permanent residence is not an option now as he overheard the interaction. Rather than prevent the departure, Momonga wishes to say his goodbyes properly to Kumoko which D permits even creating a channel so that Momonga can finally speak directly with Kumoko. Momonga and Kumoko have one last talk, with the Overlord thanking the spider for the enjoyable battle. Saying goodbye he tells Kumoko that she is welcomed back if she ever comes again. A light soon fills the room, and Kumoko and the terminal vanish leaving Momonga and his two aides alone. Albedo and Demiurge having been using time countermeasures witnessed the entire thing and voice their disappointment of failing Momonga's mission. Momonga is not sad but rather happy at their hospitality they shown their guests stating that while it was regrettable they should not give up.

Kumoko and Ariel soon return back to their world at the same exact spot and time when they vanished Their returned witnessed by the small boy with the blue muffler who swore they only vanished for a short time. Ariel finding herself back in her own world is surprised but not unhappy. The Demon Lord connects to Kumoko to have a chat with the spider who could only mutter about D. The word is not familiar to the Demon Lord and Kumoko rambles on of how the experience at Nazarick was wonderful. Seeing that the spider is ignoring her Ariel turns aggressive that Kumoko finally realizes that her once turned enemy turned friend as returned to being her enemy. Ariel states that she merely wishes to enact a punishment on Kumoko for her previous transgression against her before killing the spider in a bloody mess. The death does not kill Kumoko who is reborn via an egg she stashed away. Kumoko is relieved that she survived, but laments that she is back in this crazy world, and already misses the other world which was like a heaven to her.