Supreme Halloween

Supreme Halloween (至高のハロウィン) is a game event in Mass for the Dead that was released on October 22, 2019 to November 1, 2019.

Background
Aura Bella Fiora and Mare Bello Fiore are on the 6th Floor of the Great Tomb of Nazarick in a small area where they have cultivated various plants from the outside to supplement Nazarick's food stores. They look on with success on their crops which in this case are pumpkins. Just when the are about to order their goblin helpers to begin the harvest, Aura notices that one of the pumpkins has a weird pattern on his face. Not only that the twins realized that all the pumpkins have the same pattern, a distorted human-like face. Aura finding this strange as the crops were normal just a few days ago decides to bring one of the pumpkins to Ashurbanipal to have the protagonist examine it. Before the twins leave, Aura orders her goblin helpers to start the harvest.

Incident
Just as the two elves left the area, Homunculus Maid, Foire arrives to assist at the orders of Pestonya Shortcake Wanko. However, she finds out from the goblins that the Floor Guardians went on an errand. She asks after the goblin finish harvesting the crops what will they do next. The goblin cannot answer that as they did not receive any follow-up orders. Thinking that the elves would like something to snack on Foire decides once the harvest is done to prepare a treat.

Elsewhere Mare and Aura show the protagonist the pumpkin and ask if the pumpkin has any disease. Vaguely remember seeing something similar in a book the protagonist is assisted by the elves and Slimeko in locating material in the library. The pumpkin is set aside from their work, but so far their efforts are fruitless as they are unable to find anything under plants, so they look in other genres. While Slimeko is browsing through books, she notices that the pumpkin in her vicinity. Wondering how it got there, she thinks it to be nothing and turns her back to it. The vegetable, however, begins moving. Slimeko sensing something foreboding behind her is attacked. The other reacting to the attack, quickly come to her aid and smash the pumpkin.

The group ponders what just happened, though the protagonist finally remembers what the pumpkin reminded him of, a "Jack-o-lantern." However, it still does not explain why the pumpkins are acting this way as the two elves did not find anything abnormal when the planted and tested the soil. Realizing the danger the two rush back to the 6th Floor to warn their subordinates. Slimeko after nearly being attacked by the pumpkin contently eat its remains before she and the protagonist go to inform Momonga of the situation.

Back on the 6th Floor, the goblin servants are busying themselves harvesting the pumpkins. They are interrupted by Aura and Mare who order them to stop. Seeing that everything is fine, they think that the pumpkins are not moving as they have not ripened yet. The two make plans to burn the field to prevent the monster plants from harming anyone else. However upon hearing that the pumpkins begin to move and surround the elves and goblins. The elves battle the horde of plant monsters, easily decreasing their numbers/ Aura believes that the monsters must are actually a predator species that lure their prey using their pumpkin disguise and are intelligent. She hopes that none of the pumpkin monsters escape the field or they will have a serious infestation.

In the Round Table Room, Albedo reports to Momonga that the experiment handled by Aura and Mare is underway. Using the new field in Nazarick, it is hoped by the Overlord it will increase their self-sufficient. However, Slimkeo arrives and reports to the two of the serious circumstances regarding the latest crops from the 6th Floor. Momonga is stunned by the development and Albedo attempts to apologize for the incident though the former is more interested in stopping this potential threat. He orders Slimeko to relay a message to Aura and Mare to eliminate the creatures, but if possible to capture some for study. However he tells them to prioritize the creatures destruction should capture fail.

Unbeknownst to the rest Foire is making her way to the staff cantina with ingredient she got from the field, among them is one of the strange pumpkins. The maid is eager to help prepare something special using the pumpkin for the dark elves. Though the pumpkin looks strange she hopes it will be filling. The pumpkin moans, but the maid dismisses it as her stomach growling and continues on her way.

Meanwhile Aura and Mare are having difficulty destroying the creatures as the seem to regenerate themselves despite being smashed to bits. Slimeko and the protagonist arrive to provide assistance. Seeing the problem, the protagonist realizes something from the Halloween Festival and remembering that Slimeko had eaten one of the pumpkins, now is finding herself sick, orders her to spit it out. The slime complies, revealing the intact pumpkin. Aura and Mare seeing that they cannot destroy the creatures, lure them into an earthen dome and seal them.

The protagonist and Slimeko leave the field and return with costumes for Aura and Mare to wear. The protagonist explains the Halloween Festival and that the pumpkin monsters are not plants at all but are possessed by spirits. The For the death ritual to ensure they stay dead they must wear the costumes, which were collected from outside Nazarick and carve each pumpkin and light a flame inside. To test the method, the elves use a nearby pumpkin monster. After they paralyze, carve and light it the spirit is banished leaving only a decorative husk. Proud of the success Slimeko and the protagonist rush back to Momonga to inform him of the new while Aura and Mare deal with the issue. To make it more efficient, Mare opens a small hole in the earth dome and the siblings systematically paralyze, carve and light each pumpkin monster that comes through and leave the decorative husks out in the field.

By the time Slimeko and the protagonist return, they find the twins effectively remove the threat of the monsters. However their victory is interrupted when Yuri Alpha reports that more of the Pumpkin Monsters have made their way to 5th Floor. The twins and their comrades are surprised that the monsters made their way there as they quarantined the fields to prevent their monsters form escaping. They assume that the pumpkins escaped prior to the quarantine and the group decide to head to the 5th Floor before things worsen.

On the 5th Floor, Cocytus is having trouble with the pumpkins as they are regenerating themselves despite his constant attacks. The only way he managed to keep them from spreading was by corralling them in ice. The group from the 6th Floor arrive to lend their expertise and the pumpkins soon find themselves butchered into jack-o-lanterns. However something strange occurs and the remaining pumpkin monsters begin eating one another, growing in size as a single entity until it creates a Giant Pumpkin Monster. Faced with this new adversary, the group prepare to fight ans slay it. Once the monster is defeated, Aura laments that they were unable to capture it, but decides to let her failure go and consider that Nazarick is now safe.

Foire is at the cantina looking for the Head Chef and Sous-chef. Sadly both are absent so she decides to write a note to them to prepare the pumpkin she brought. The pumpkin she brought with her begins to move on its own and quietly approaches her. Just when the maid realizes the danger it attacks and launches itself at her. Reporting back to Momonga and Albedo in the Roundtable, the dark elves, insect, slime and protagonist confirm the destruction of all the pumpkin monsters in Nazarick. They are interrupted by a voice at the door to the room. Momong recognizing it to be Foire allows her to enter. The occupants of the room are greeted by a strange sight, a muscular Fiore with a pumpkin attached to her head. Briefly stunned by the sight, Momonga orders the Floor Guardians to take care of this threat. Forcing the possessed Foire into the hallway the struggle to safely remove the pumpkin form the maid without harming her but it proves difficult. Momonga furious that such a creature would do this to Herohero's creations, orders them to use necessary force. Cocytus launches an attack that causes the pumpkin to react in pain and to the surprise of many buffs up in strength. Now understanding that they needed to overwhelm it with power, Momonga orders Aura and Mare to prepare to hollow out the pumpkin. The group succeed and Foire is freed from it control. After attending to Foire and ensuring no lingering possession is present, Demiurge who has analyzed the corpses of the monsters confirms that the monsters were a result of contamination of various magics, both from the New World and also due to Mare's experiment to assist in the crops' growth. Momonga rather than halt the food cultivation experiment, at the encouragement of Demiurge, decides to continue. This time though with proper safety measures and a new crop: pumpkin squash.

Now that the crisis is avert, Momonga decides to teach the Floor Guardians about Halloween. To start he asks the protagonist what information is available on the holiday, which is a spell incantation "Trick or Treat".

Aftermath
That night Momonga finds himself in his office when he is greeted by a knock at his door, where Mare and Aura are still in their costumes declaring "Trick or Treat" for Nazarick's Halloween celebration. Eager to comply, Momonga delivers each of them a pile of goods for them. Thanking them, the twins eagerly continue to the next room for their trick-or-treating, bumping into Shalltear Bloodfallen and commenting on her revealing costume. Momonga realizing that some of the female Floor Guardians may have misunderstood the purpose of the holiday decides to make himself absent for the night.