Life in a Different World with the King of Immortals (不死者の王との異世界生活) is a collaboration game event in Mass for the Dead with characters from Overlord and Re:Zero. It was released on January 29, 2020 to February 14, 2020. The event was re-released on March 4, 2021 to March 18, 2021.
A summon experiment using the Crack in the Throne Room and the chaos stones. Before Momonga, four people and one from another world were summoned. One of them, a black-haired boy, was found to have a particular constitution--- Natsuki Subaru's new life in a different world begins.
Natsuki Subaru along with his comrades, Emilia, Rem, Ram, and Puck after visiting Irlam Village make their way back to Roswaal's Mansion. During their journey through the forest, a thick fog envelops the area. Subaru and his friends believe it to be nothing soon are grabbed by a mysterious gust of wind, that spirits them away. Soon after they are gone, the fog vanishes and only silence remains in the forest.
In the ruined Throne Room, Demiurge and Momonga are conducting a summoning experiment, using the Crack. Using a Chaos Stone as a catalyst, the Crack glows eerily and erupted in a brilliant flare of violet light, in which four human bodies are spewed from its mouth. Subaru's group get their bearings together only to see the demon and undead in the chamber. Rem gets into a defensive stance and orders Subaru to get behind her. However, Momonga states that they do not wish to harm them. He tells them they are in his domain, the Great Tomb of Nazarick, and wishes if they could peacefully exchange information.
Subaru though terrified at the sight of the skeletons and a Death Knight maintains a courageous face, in the presence of the other women, and introduces himself to the ruler of Nazarick. Momonga explains to the group, that they are no longer in the Lugnica, but were summoned to another world via an experiment. Likewise from Subaru tells him that the world they were pulled from was a fantasy world similar to the new world Nazarick is located in. Bottom line from Momonga's hypothesis they are unable to return back to their world, as they do not quite understand the laws of physics regarding the Cracks. Subaru worries about his [Return by Death] ability, as it make be connected to the world he was originally transported to. He is afraid without it he will definitely die. Subaru heart pounds, and he clenches his chest as he collapses. His friends worry on his sudden fatigue, but Subaru tells them not to worry. Thinking to himself that he felt the penalty of his ability, it confirms that the [Return by Death] was still present as the Witch's hand effect had activated.
Momonga is more concerned about Subaru's health and offers to give the boy a room so he may recuperate. A skeleton and Death Knight present in the room begin to move toward Subaru, despite Momonga never giving them an order to do so, and begin aggressively moaning at the boy. Rem seeing her friend being threatened, immediately puts herself between them and Subaru. She lands a few blows with her chain ball on the Death Knight with a negligible effect. The incident is put to an end by Momonga who used [Napalm] on the undead to bring it back under control. The Overlord sincerely apologizes for the unprovoked attacked. Subaru seeing the display of power and Momonga willingness to attack a subordinate finds it concerning, but thanks the undead nonetheless.
After the two undead are confined, Demiurge humbly apologizes for the outburst of the summons under his watch, but Momonga states it was his responsibility. He wonders what caused the undead to be set off in a savage manner, and turns to his guests if they might known. Ram tells them they don't know but perhaps they should asks Natsuki Subaru for answers. Subaru knowing that he cannot reveal his aura attracts demon beasts due to the penalties if he revealed the nature of [Return by Death] to anyone. He lies that it was his scent that attracts magical beasts, which unfortunately was caused by his sick disposition. Momonga admits that the boy's appearance is poor, though the reason is still shaky, he accepts it at face value. Since there are many magical beings within Nazarick, Momonga orders Demiurge to guide the group to a vacant room on the 9th Floor. After the Floor Guardian and Subaru's group leave the Throne Room, Momonga ponders on Subaru's condition that can attract demon beasts.
In a common room, Demiurge allows the members of Emilia's Camp to make themselves comfortable as he departs. Subaru immediately flops to the floor in exhaustion. The girls scold the young man for drawing attention to himself. Turning to the grandeur of the room, they comment on the financial wealth of their host. Rem is more concerned with Momonga's power, comparing his magical ability to equal or exceeding Roswaal L Mathers. Even Puck admits he cannot divine the limits of Momonga's power and recommends not agitating the undead. Subaru is concerned that Momonga planned the attack on the undead to gain their favor. Rem states that the should focus on returning home as Emilia's Royal Election will happen soon. Puck believes though this world is different from their world, it should still be possible to return home using Beatrice's [Door Crossing], given the doors of her library can open to any location unless certain conditions are met. He believes that by studying the Cracks that they may be able to find a clue to return home. Subaru agrees with Puck's assessment just as there is a knock at the door.
Solution Epsilon comes into the room and introduces herself as a member of the Pleiades that has been assigned to protect them while they stay in Nazarick. Subaru is cautious on the "protection," but Solution states its a precautionary measure given the previous incident with the summons and also to prevent someone from his party from being eaten. After the maid leaves, Puck confirms through his empathy, that she is not lying though warns Subaru to never be alone with the woman. For the meantime, the group decides to follow the maid.
Under the guidance of Solution the group is taken to the edge of a large forest located near Nazarick. Solution in her adventurer attire brings them to Aura Bella Fiora. Emilia is surprised by the new character and blushes. Aura explains that Momonga wishes to confirm if Subaru's smell truly attracts demon beasts to his location and this arranged for a little experiment. Noticing Emilia muttering to herself while staring at her, Aura asks if everything is alright. Emilia explains that she a half-Elf and is just excited at meeting another elf. Aura is a bit confused, and she explains that she and her brother are dark elves, so the comparison doesn't really matter to them. Seeing Emilia disappointed face in the lack of Aura enthusiasm causes Aura to say that it's okay since they are all elves, cheering her up. Subaru and Puck watching Emilia make a new friend joke on her progress, causing her to be annoyed by their jest.
Returning to the matter at hand, Aura explains she believes Subaru's ability will hinder Nazarick operation, thus Momonga has decided to limit the group's freedom. Some experiments have been planned to confirm some factors, such as range and influence. Subaru queries on the return of his freedom, which Solution replies that Momonga will mercifully free Subaru once they confirm the effect and calculate the research's value. Having no choice to refuse, Subaru decides to proceed. Noticing the maid's change of dress, he inquires about it, only for the maid to tell him its what she wears when outside of Nazarick. To make things clear if any other adventurers come in the area, her name is Soi.
Subaru asks how will the experiment work. Heavy footsteps are heard and a rodent-like creature soon emerges from the forest. Aura introduces the beast as Hamsuke and her pet. Hamsuke attempts to greet the gathered, but suddenly becomes feral. Roaring she singles out Subaru, who then runs for his life with the magical beast on his tail. Aura sees that the scent from Subaru has affected Hamsuke, and appears that the condition affects creatures no matter their level of intelligence. Needing to stop her pet from chasing the latest asset of Nazraick, Emilia offers to take care of it and uses her magic to cast ice on the hamster. The cold temperature causes Hamsuke regains her senses. She sincerely asks for forgiveness to Emilia's camp, especially to Subaru, for her actions. Aura chastises the "Wise King of the Forest" for losing her head. Subaru thinks such a title is unsuitable for Hamsuke, though Puck points out that he is called a Great Spirit, but he doesn't live up to it in appearance. Ram also comments Subaru's titles are underwhelming, causing the young man to be emotionally hurt.
The event is enough to convince the two agents of Nazraick that Subaru's ability to attract monsters is genuine. Aura believes they must test the range of the ability. Just as if on cue, a pack of savage wolves burst from the forest. The dark elf comments that these beasts shouldn't be anywhere near the area, since Hamsuke would have scared them away. This affirms that the range of Subaru's "scent" is wider than expected. Hamsuke retracts her claws and decides to take care of this herself.
After repulsing the beasts that approached the area, Aura and Solution continued conducting various experiments and confirmations on Subaru's ability until they were satisfied with their findings. Subaru is glad the testing is over. Though Emilia apologizes for her inability to protect her friends. Rem noticing Subaru is hurt, uses her magic to heal him. The magic surprises Solution as magic scrolls were normally used.
The group returns to Nazarick, where Solution and Aura give their reports to Momonga. Besides the evaluation of Subaru's ability, Aura gives her master her assessment of Emilia's power, believing it to be stronger than originally believed. Solution also admits that the girl's powers are unknown as is the spirit she is contracted with. Momonga who had been observing the experiment via the Mirror of Remote Viewing agrees with the two reports but urges they need to trend on the side of caution. Aura suggests using Subaru's ability to their advantage especially in flushing out potential dangerous creatures.
In the Throne Room, Demiurge is showing Emilia's Camp the Crack. He goes on to explain that the Crack is a summoning phenomenon that Nazarick has manage to utilize to their advantage using Chaos Stones from defeated Chaos Beasts. From their experiments many things have appeared from their Crack and so far have been obedient to Nazarick. Subaru and co. are exceptions to this experiment. Subaru asks if it is possible for them to return to their world via the Crack. Demurige states that to be impossible, and demonstrates by having a skeleton approach the Crack and cross through only to vanish. The demon elaborates any object or being that crosses in the Crack will immediately vanish, and any magical sensory and connection to the being is immediately severed. While they cannot confirm whether if a being that crosses the Crack dies, but so far none of their skeletons have ever returned back making it unlikely that the Crack is connected to Subaru's world. Demiurge states that he is still researching the Crack and promises to release any new information that comes from their research.
Subaru is frustrated on the lack of a definitive way to return home. Rem seeing his tension, urges Subaru to be patient and that they should cooperate with Nazarick until a solution is found. Demiurge warns them that soon the scheduled time for the Crack to unleash Chaos Beasts will soon come and asks them to be prepare to eliminate the threats.
After the Chaos Beasts are subjugated, Subaru and his friends retire to their room. While the girls retire to the baths, Subaru is fuming on Ram's inability to use mana properly. Without Roswaal's treatment of infusions she cannot absorb and discharge magic as normal. He wants to find a way back to their world, but does not wish to rely on Momonga for aid. Emilia returns to the room, to offer some comfort to Subaru and his inner turmoil. She states that Demiurge offered to provide a treatment for Ram's infusion mana so everything should be fine. Though Subaru worries about the upcoming Royal Election, and believes that if they don't find a way back, Emilia will lose her chance to become Queen. Emilia states that Subaru shouldn't be worrying about it and confesses that she would probably not be elected as Queen. Subaru tries to explain his real reasons for returning back to their world, but hesitates, afraid that the penalty for [Return by Death] may still apply. Emilia still has faith that if they work together with Momonga, everything will be fine. Subaru agrees with her advice, and that all hope is not lost in their situation. Rem comes in asking Emilia that the others are waiting for her in the baths. After his talk with Emilia, Subaru renews his determination of the next day in Nazarick.
On the 6th Floor of Nazarick, the group meet Cocytus, who Emilia is delighting that the two share the same specialties in ice magic. Subaru finds it endearing that Emilia is making new friends. Narberal Gamma and Ram also bond over their mutual dislike of inferior beings, namely Subaru, much to the main protagonist's chagrin. Ram's mana infusion problem has been solved in part due to Sebas Tian's skill in Ki. The group happily start assisting Nazarick in clearing Chaos Beasts on the 6th Floor.
In the party's room, Rem gets into an argument with one of the Homunculus Maids, Foire over cleaning the room. Rem insists that since she is a maid that she is more than qualified to take care the cleaning for Subaru and Emilia. Though Foire states its the duty of the host to handle such trivial matters. Pestonya Shortcake Wanko intervenes in the argument and suggests they resolve it through a contest. Both maids will display their skills by cleaning the room. The two maids agree, but what was suppose to be a competition ends up as a collaborative effort, leaving both maids impressed with each other's skills.
Subaru is still concerned that its been a week and still a method to return to their home dimension has still not been found. They receive a visitor at their door, revealing to be a male dark elf. Emilia recognizes the elf to be Mare Bello Fiore, the twin brother of Aura. Mare greets them as he comes into the room with a request for tomorrow's mission in the wilderness outside Nazarick.
The next day, in a dry plains area, Subaru is helping the two dark elves fish for a specific monster: Giant Worms. Aura wants to capture some specimens for Nazarick as they can be used to make tunnels underground. To fish for them from the ground, Aura wants to use Subaru's monster attracting ability to draw them out. Emilia and the others, understanding the two want to use Subaru to act as bait, object to this dangerous task. The two elves assure them everything will be fine and that the boy will be perfectly safe. Ultimately, Subaru makes the decision to be used as bait for his friends. Rem gives Subaru the end of her chain ball to hold unto so she could draw him out of danger in case of monsters attacking.
Several hours after the "Giant Worm Fishing" operation has begun, so far the group has only caught small specimens. Aura wonders if Subaru's scent is not reaching the worms deep in the earth, and perhaps maybe they need a smell of something stronger. She recommends Subaru to run around to sweat more to attract them. Subaru' knowing full well that his "sweat" is not the catalyst for attracting monsters plays along and runs around. No monsters are in sight, so Subaru calls Emilia over to tell her a certain secret. Just ash he was about to speak about his [Return by Death], its penalties activate. Feeling the overbearing force on his heart, Subaru tells her its nothing and walks away from her. The ground starts shaking indicating that something in digging its way towards them through the earth. Aura tells them to get ready as the Giant Worm is coming. Just before the earth explodes below Subaru's feet, Rem pulls him out of harms way and Aura grins at finally capturing a worthy prey. All this is watched by Momonga via a Mirror of Remote Viewing.
In Nazarick, Momonga calls Natsuki Subaru alone to the Round Table Room. First Momonga thanks the boy in helping Aura and Mare capture a Giant Worm. Though the real reason why he called Subaru here was so he could privately talk to him about his "condition" that he is hiding. Momonga explains why he suspects Subaru is hiding something about the truth of his ability. He took note that whenever Subaru placed himself in a dangerous situation, he was not afraid of putting his life in danger, the "Giant Worm Fishing" proved that. But his personality hints that the boy sees more death and pain than an average individual. Momonga suspects Subaru is hiding this from his friends and he is asks if there is something more to his ability concerning his heart. Subaru neither confirms or denies Momonga's musing, but it is enough for the latter to confirm something is wrong. Momonga wonders if there is something holding Subaru back from speaking the whole truth. He tells Subaru that if there is no way to go any deeper, then he won't pry out of respect. Subaru asks what he should do with his condition. Momonga tells him that while he does not know what exactly plagues Subaru, he can tell it is a great burden on him, but urges him to endure it.
Subaru finally finding someone to empathize with on his condition silently cries. Momonga seeing Subaru's tears, consoles him and asks if Subaru would like to give up and stay in Nazarick. Subaru is surprised by the offer, though Momonga confesses he is more interested in Subaru's condition and believes that he can help him deal with it. Subaru is thankful for the offer, but politely declines over various reasons: Ram would never admit to settling in the dungeon without seeing Roswaal and her sadness would pass to her sister Rem, and Emilia needs to return to the their world to participate in the Royal Election. Subaru expresses that he wishes to support Emilia in becoming Queen. Momonga seeing Subaru's loyalty to his comrades admits they are good reasons to return back.
Momonga tells Subaru to call his friends waiting outside the room. Once they are inside the room Momonga announces his gratitude towards them for their help. He fully expresses to keep his promise with the group in finding a way to send them home when conditions are present. The entire group is thankful for Momonga's generosity. Just as Subaru thanks Momonga, he soon experiences a penalty, despite not having revealed his [Return by Death] to anyone. The room goes dark and a swirling purple mist engulfs the room. The others in the room ask if Subaru sees anything as they are unable to see anything. Subaru though sees a vague figure reaching out to him in the mist. Realizing that something is about to happen, Subaru tells Emilia and his friends to grab his hand. Puck sensing the urgency tells his contractor and the others to obey Subaru. The group hold hands and soon disappear from Nazarick.
Momonga who had witnessed the strange event, ponders on the group's disappearance to be the work of the entity holding Subaru's life in its grasp. He reasons that it must have took Subaru and his friends away just as they were beginning to trust each other. Though he reasons it could be a time limit on summoning, Momonga suspects that Subaru and his comrades will return to their world, and laments that the boy is burdened with such troublesome things. Momonga then urges Subaru to not lose.
Emilia's Camp soon finds themselves back in the forest between Roswaal's mansion and the Irlam Village. They are unsure on how they returned back to their world, but are thankful that everything worked out. Emilia is sad that they couldn't say goodbye to their new friends. Rem is concerned about the amount of time that has passed and wants to return to the mansion immediately. The other girls hurry back to the mansion, leaving Subaru behind, who thinks about Momonga telling him he supports him. Subaru's mutely agrees and states that he will do his best as he hurries to follow his friends.