2020 New Year – Advent of Shiramochi-ō

2020 New Year–Advent of Shiramochi-ō (2020正月-白餅王降臨-) is a holiday event that occurred in the Great Tomb of Nazarick. It was released on December 30, 2019 to January 9, 2020. The event was re-released on December 28, 2020 to January 15, 2021.

Background
Momonga visits the Round Table Room for a meeting and finds Albedo and Demiurge relaxing by a kotatsu and is confused. When he looks at it closely, a mysterious New Year mood has spread in Nazarick. Momonga set out to restore Nazarick.

Incident
Demiurge and his Evil Lords were conducting an investigation of the ruined Throne Room trying to make sense of the Crack on where the Throne of Kings once stood. After spending time speculating on why the Crack on the 10th Floor is the only anomaly that allows them to summon Chaos Beasts, it beings to glow despite it still being a ways for a scheduled Chaos Beast cycle. Demiurge deduces that something is being summoned and sure enough, a strange monster resembling a Kagami mochi appears. Demiurge tries to use his mental magic, [Command Mantra] on it however to the demons' shock it is immune to his spell. The Floor Guardian deduces that the creature is a Summon and under the control of a powerful Summoner. He orders his demons to destroy the threat in Nazarick, which proves tricky as the monster is not an easy foe to defeat with. When the last blow is struck the monster begins to glow and the entire room in engulfed in light.

Momonga, who visiting the Round Table Room for a meeting, finds Albedo and Demiurge hanging there in with a kotatsu and is confused. He investigates and discovers that a mysterious New Year mood has spread through Nazarick. Disturbed by the lax behavior of his Floor Guardians, Momonga conducts his own investigation and summons several Death Knights and Skeletons to search for any clues to the source of this New Year's mood.

Shalltear Bloodfallen is recruited as it seems the New Year's mood does not affect undead beings. However when she greets him a Happy New Year, he is proven wrong and proves his theory that something is affecting the minds of those in Nazarick. He accompanies her to the 6th Floor where they find the Mare Bello Fiore and Aura Bella Fiora dressed in New Year kimonos playing Hanetsuki. Though everything seems okay, a fissure appears and Chaos Beasts emerge along with a new monster that resembles a kadomatsu. Easily dispatching them with the help of the Floor Guardians, Momonga wonders what type of monster the kadomatsu was. A Death Knight reports that two of its agents have returned with no abnormalities, making Momonga sure that whatever is happening is only affecting the dungeon. Deciding to go to the 1st Floor alone, he greets the protagonist, Slimeko and Solution Epsilon that have returned. He nearly loses composure when Slimeko wishes him a Happy New Year as it shatters his belief that the New Year phenomena is the only epidemic to the tomb. The protagonist explains that not to be the case as it seems only that the strange New Year's mood was settling only in Nazarick. It appears whatever is happening is spreading and the group now see the seriousness of the situation. From what Momonga could tell, other than that strange Chaos Beast that appeared the New Year phenomena seems only to be in the dungeon. As of now the only ones who have not yet been fully impaired by the mood are the ones he gathered. Solution attempts to apologize for this weakness, and her sisters. But Momonga states that this is not her fault, but should count themselves fortunate that the ones who were outside Nazarick have been least affected. Faced with this Momonga decides to recall Sebas Tian and other summons until they can determine what is going on.

After awhile Momonga receives a report from his undead forces surveying the floors. They report that everyone has been accounted for save for Cocytus and the Pleiades. The protagonist having passed by Cocytus, recalls that the Floor Guardian mentioned about visiting a shrine and was heading towards the Fake Nazarick. Sure enough a strange tori is found on the 1st Floor of the Fake Nazraick. It is determined to be a Hatsumōde and after sending several skeletons to and back Momonga confirms that there is something on the other side. The protagonist believes that from the shadows casted by the gate, it to be some sort of unique Chaos Zone and possibly the source of the New Year atmosphere that is spreading around.

Momonga and his allies enter it and find that the interior of the fake dungeon has bee radically changed. Instead of a perfect replica of Nazarick, they are faced with a desolate misty desert. Solution scouts the area and so far despite the looming eerie feeling finds no danger but gas encountered Entoma Vasilissa Zeta, baking New Year's confections. Unsure if Entoma is compromised the group decides to greet her. Entoma welcomes them and explains that she was making mochi, which Momonga finds odd as she is not supposed to have a Cook class and surmises that the phenomena is responsible for giving her the skill. The party and Momonga soon encounter a Mochi Type Beast. After killing it to their surprise it leaves behind mochi similar to a drop item. Entoma unperturbed at how they defeated the Chaos Beasts thanks them and asks if they can collect more mochi for her to bake. Momonga decides to play along with the charade and complies as does his allies as they collect the mochi by killing the nearby mochi monsters. Once they are done Entoma offers them green mochi, made from the mochi they collected and her green bread. While Slimeko is eager for the treat, Momonga and the protagonist advise her not to eat it, unsure of the effects of eating food in the strange area. Not to be rude to Entoma, Momonga asks her to doggy-bag it for them and they make their way to the next floor. While traveling Slmeko hears a sound similar to a gunshot the party follows it deeper into the misty dimension. Solution scouts ahead only to return to report she found CZ2I28 Delta acting odd in that she shooting a metal plate in the air. Momonga is left confused and decides to see for himself, where Shizu is doing as Solution reported. Momonga asks if she is targeting an enemy only for the Automaton to state she is playing with the wind and continues to shoot in the distance as it's fun. It's apparent that Shizu is affected in some way though different compared to the other NPCs, though Momonga guesses that her Automaton race could be a factor. The group then runs into a Kadomatsu Type Beast and swiftly defeat it. They are attacked by a mochi monster, which they kill and are surprised when Entoma gleefully eats the leftover mochi pieces. When questioned about what transpired Entoma believes that they were only collecting mochi, making Momonga ponder on the mental effect of the New Year's atmosphere as his influenced subordinates are not perceiving these new Chaos Beasts as threats. The group realizes that the effect of the phenomena seems harmless though the summoned monsters only appear to take action when interlopers interfere with the holiday atmosphere. Since they are unable to locate a transfer gate, they reason that something is purposely leading them to some goal. The group defeat the two and are allowed to pass where they meet with Yuri Alpha and Lupusregina Beta.

They come across a shrine where Cocytus and Narberal Gamma welcome them to the Takemikazuchi Shrine. They explain behind the shrine is a path leading to the 10th Floor. Momonga wishes to pass but the two to his surprise block his back insisting they duel them. Cocytus and Narberal are unable to ignore their directive and Momonga and the others have no choice but to fight as they are under some influence. The group defeat the two and are allowed to pass where they meet with Yuri Alpha and Lupusregina Beta. When Yuri is asked why she left her post, Yuri replies that it was a holiday. All in all Yuri and Lupusregina appear to be mildly affected by the New Year atmosphere. Though they invite him write poetry though they are suddenly attacked by Chaos Beasts. After dispatching the monster the groups continue to participate in the festivities. Solution decides to scout ahead once again, and Momonga suspects that the "final boss" is awaiting them on the 10th Floor. To be safe Momonga plans to send in the Skeletons to gauge the strength of the enemy.

In the Throne Room, the group find a bunch of Chaos Beasts by the throne. Momonga orders his summons to eliminate the intruders. When the last enemy falls, a mysterious being appears and proceeds to annihilate the skeleton party. Momonga observing the battle through his summons until their defeat analyzes the entity as a strong Chaos Beast and deduces that it is unable to leave the Throne Room. Planning an all-out attack, Momonga and his allies cross the Gate. The entity introduces himself as Shiramochi-ō, before Momonga starts by using [Reality Slash] but the being remains unfazed. Momonga notices the entity's lack of personal anger. He surmises that the White Advent King lacks an actual will and acts similar to a raid boss. That being said, the group begins to battle the entity, injuring it to a point where Shiramochi-ō is forced to use area attacks.

Aftermath
In the end, Shiramochi-ō is defeated and Nazarick is returned to normal. Returning to the Round Table Room Momonga finds Albedo and Demiurge back to their usual selves. They discuss the events that took place and debate whether it was an intentional attack and the connection to the New Year. Momonga postulates that due to the rules of the New World, it was probably some random phenomena that was summoned via the Throne of Kings. In the end, they may never know, though Momonga cannot help with how uncannily the situation they experienced was just like a game event. Demiurge promises to research the event. All in all the event allowed Nazarick to celebrate New Year's and was written off as a fun adventure.