Bloody Valkyrie Incident

The Bloody Valkyrie Incident was a battle between Ainz Ooal Gown and a brainwashed Shalltear Bloodfallen.

Background
During a mission undertaken by Shalltear Bloodfallen, the Floor Guardian caused a disturbance, annihilating an iron ranked adventurer team. One of the adventurers managed to slip away and escape back to E-Rantel. The Guildmaster Pluton Ainzach learned from the survivor that the team had faced a powerful vampire with abnormal abilities. Usually, a platinum ranked adventurer would suffice to eliminate the monster, but given the circumstances, Pluton decided the best course of action would be to summon all mithril ranked adventurers and handle the situation from there.

Incident
Ainz Ooal Gown upon learning that a Shalltear had rebelled against Nazarick, quickly teleported back to the Great Tomb of Nazarick. Reuniting with Albedo they use the dungeon's administration system and determine that Shalltear is indeed under the control of an outside influence. In order to find Shalltear, he and Albedo go to the Frozen Prison on the 5th Floor to visit Nigredo and use her magic to locate her.

Through Nigredo's magic, the pair discover that Shalltear is maintaining her position but is now fully equipped in her Bloody Valkyrie form. Soon after before they could commit any action, Ainz then learns from Narberal Gamma that the Adventurer's Guild in E-Rantel is looking for him concerning a vampire. Resuming his identity as Momon, and goes to the Adventurer's Guild meeting. The meeting had summoned several other mithril adventurer teams: Darkness, Rainbow, Sky Wolf, and Kralgra. The plan was to have the adventurers combine their forces to defeat the vampire. Knowing this would be detrimental should his connection with Shalltear be discovered, Momon interjects stating that they would all die.

Momon insists to the Guild leaders, E-Rantel's authorities and his fellow Mythril adventurers that he will exterminate the vampire. He concocts a story that tells him he is familiar with the vampire, as he has been hunting her, Honyopenyoko, and another vampire. All of the people in the room are aware of Momon's reputation but are doubtful that he alone could defeat the vampire.

Their concerns are washed away when Momon reveals his trump card: a Sealing Crystal containing a seventh tier level spell. Momon insists that Darkness will fight against Honyopenyoko alone, arguing that involving others in the fight will simply increase the number of casualties. Nearly all in the room acquiesce to his terms save Igvarge of Kralgra, who demands his group accompany Momon in the fight against the vampire, which the latter accepts. En route to the destination where Shalltear is located, Ainz has Kralgra ambushed, intending to silence them and blame their deaths on Honyopenyoko. Igvarge manages to slip away but soon is restrained and killed by Mare Bello Fiore. Witnesses being taken care of, Ainz introduces Hamsuke to Albedo and orders Narberal to escort the hamster to Nazarick so it may be acclimated with the members of the tomb.

Albedo and Ainz head towards Shalltear. Just as Nigredo magic shown she was indeed on standby possibly due to an incomplete mind control magic. Despite having countermeasures against mind=control the vampire has fallen under an influence of at least as strong as a Divine Item. To undo the spell on Shalltear, Ainz reveals his wish-granting magic ring, Shooting Star. To his shock, the magic fails. Ainz frantically orders a retreat, teleporting himself and Albedo to a safer location. Before Albedo asks what is wrong, however, Ainz goes through a tantrum of rage kicking the dirt in anger. When his undead racial effect kicks in, the Overlord calms down and tells Albedo the magic of a Divine Item didn't have any effect on the mind control placed on Shalltear. In that effect, it could only mean one thing that Shalltear was affected by a level greater than it: World Items. Ainz fear is justified as they are an object of power that could change the balance of power. He didn't anticipate any World Items to exist in the New World. The potential danger of the World Items brought to light, Ainz first orders Albedo to recall all Floor Guardians outside of Nazarick to verify if they are comprised and to escort him to the Treasury.