White Sliver and the Undead King

Death Ruler and Berserker is a collaboration game event in Mass for the Dead with characters from Overlord and Youji Senki. It was released on November 8, 2019 to November 22, 2019.

Background
A summoning phenomenon caused by the stones embedded in the Throne of Kings and the stones dropped from the Chaos Beasts. In front of Momonga, who witnessed the verification experiment, six new visitors from another world appear. The Imperial 203rd Aerial Mage Battalion belonging to the Imperial Army - a military unit headed by a "little girl."

Incident
Maj. von Degurechaff and the members of the Imperial 203rd Aerial Mage Battalion are currently in the air testing out the Elenium Type 97 ‘Mobile Assault’ Operation Orb for Professor Adelheid von Schugel. During Tanya's activation of the magic equipment, her orb malfunction and it at the verge of a meltdown. Seeing her soldiers around her, she orders them to distance themselves from her to save themselves. The others are conflicted to what to do when a Rift suddenly appears and sucks the Major followed by the rest of her team into its grip. The soldiers land in the middle of the ruined Throne Room on the 10th Floor of the Great Tomb of Nazarick. Recovering slightly they find themselves amongst a troop of Skeleton Warriors, led by a demon and a robed undead. Tanya upon seeing the monsters originally thinks perhaps they are costumes and at an amusements park in the Unified States, however changes her ind when she notices the realistic qualities of the so-called costumes. The members of the battalion tense and grip their weapons at the confusion of their new surroundings. The robed undead says that the summoning was successful though wonders if the group would be sent back. The undead introduces himself as Momonga, and request who in the group speaks for them. Initially mistaking Matheus Johan Weiss as the leader, followed by the denials of Wilibald Koenig, Rhiner Neumann, and Vooren Grantz, Momonga comes to Viktoriya Ivanovna Serebryakov who directs him to Tanya von Degurechaff. Momonga is surprised as Tanya is just a little girl.

The two leaders then discuss the current circumstances they find themselves in. Momonga after hearing Tanya's story of how her team came through a Rift is certain acted as a doorway to another world and quite different to the one that it previously connected to given the level of technology and culture. Likewise Tana thinks to herself and suspects this all to be Being X's meddling. Momonga decides to allow Tanya's group to stay within Nazarick for the time being, in hopes of utilizing their unique skills for Nazarick's benefit. To ensure her soldiers' safety Tanya decides to accept the hospitality offered by the Overlord while they figure out a way to return home. Momonga makes it clear that their stay in Nazarick is by no means free of charge. In return for Nazarick's protection Momonga expects Tanya's team to assist the dungeon in subjugating Chaos Beasts. Tanya however informs him that his conditions may be unacceptable, as under military law her unit is to allowed to commit military action and submit themselves to another sovereign entity for. Her reason is a for of legality that even convinces Demiurge to agree as such non-compliance of the local law would be acceptable. The demon suggests a compromise, rather than the battalion provide military aid, terms of the conidtions can be changed to "hunting" rather than "battling" Chaos Beasts. In this way Tanya can circumvent their regulation without breaking their code despite it being only a degree of semantics. During the negotiations on the conditions of her team's employment as exterminators for pests, Tanya and Momonga analyzing each of their negotiations strategies, realize they are dealing with a former Japanese salaryman.

The mage battalion are sent to the 6th Floor, where they are astounded by the forest and sky. Seeing the open air, some of the members of Tanya's battalion suggest they take the chance to escape. Weiss motions against that action as he suspects that the monsters of Nazarick wouldn't allow them this opportunity if they were not confident on recapturing them. Tanya also is suspicious of the sky and wonders if its fake and ultimate orders her men to carry out their duties as they agreed with Momonga to maintain cooperation. The battalion are soon joined by Yuri Alpha who is assigned to guide them to a location in the forest on the 6th Floor where a crowd of Chaos Beasts will emerge.

Aftermath
To Be Added...