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 mind 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 Tanya 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. Tanya orders her battalion to get into formation and advance. The group engage the Chaos Beasts with effectiveness alongside Yuri. Tanya notes that while the Chaos Beasts are fearsome beasts in this world are no match for her soldiers. She is equally impressed by Yuri who uses her bare hands to smash several opponents and wonders if the executives of Nazarick have similar strength and abilities.

Her query is later answered when Momonga comes to assist and casts [Reality Slash] taking out several Chaos Beasts in one shot stunning the commander. Momonga who had been observing the battle was interested in the abilities her soldiers displayed, in that they performed aerial attacks through the trees and their projectile magic, and wanted to learn more. Tanya suspects that Momonga intentional allowed her battalion to battle the Chaos Beasts and then took the opportunity to display his power as a psychological tactic to prevent her from betraying Momonga. Tanya subtly thanks him for the assist and the two share unspoken words to convey hidden meaning of the need for cooperation so long as their goals align. Seeing that they understand one another Momonga allows his military contractors to proceed with their duties.

A day after Momonga had observed the battle between Tanya's unit and the Chaos Beasts, Momonga is with Albedo to discuss their new allies. Momonga finds the mage unit to be interesting, conveys to the Overseer Guardians that his impression of the unit, particularly Tanya, is that she is a "battle maniac", similar to Shalltear Bloodfallen. While the Imperial 203rd Aerial Mage Battalion would eventually return to their world, if the previous summoning serves as a example, Momonga wishes to obtain more information as soon as possible. Thus he decides to arrange a meeting with Tanya von Degurechaff and have Demiurge present.

While the major is attending a meeting with Momonga, the 203rd Air Mage Battalion at waiting in the Lemegeton to prepare to do another sweep to wipe out the Chaos Beasts in the Throne Room. Grantz wonders if Tanya will be fine alone with the monsters, but others like Koenig and Neumann think they should be more worried about themselves. Weiss tells his men to keep their cool and take comfort that if they die they will go to Valhalla. Narberal Gamma soon comes with Slimeko and the protagonist to tag along on their operation. Seeing they are all present Weiss takes command and leads the battalion to the next hunt.

In the Round Table Room, Tanya is explaining the concept of the Operation Orb and its function to Momonga, Albedo and Demiurge. While she is forbidden from providing information that conflicts with the military code, she is able to give them a general scope of her world's technology. Demiurge is fascinated by the magical tool and Momonga is impressed with her advanced education as he thinks she is on par with some of the Floor Guardians in intelligence. Tanya merely states it is a result of her training as an specialist officer. Momonga comments that it is a shame that her intelligence is waste at the front lines, which causes Tanya to have an internal fit.

A few days after Tanya and the other were summoned to the Great Tomb of Nazarick, the battalion are once again on the 6th Floor on another hunt for Chaos Beasts.

Aftermath
To Be Added...