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.

At the entrance to the Treasury, Albedo and Ainz are joined by Yuri Alpha and CZ2128 Delta. Before the entrance, Ainz opens it using the universal password. Passing numberless artifacts and weapons they reach the inner sanctum where the group encounters a strange octopus humanoid. Albedo recognizes it as her creator Tabula Smaragdina. However, upon further scrutiny she quickly sees it as an impostor, causing Yuri and Shizu to prepare for battle. Ainz orders them to stand down and reveals that the impostor is really a Doppelgänger named Pandora's Actor, the Area Guardian of the Treasury. The Area Guardian resuming his natural form dramatically introduces himself and offers his services. Seeing that his master has come to the Treasury, the Area Guardian deduces that he has come for the World Items and that there is a situation that may threaten Nazarick. While painfully listening to his creation's antics, Ainz pulls Pandora's Actor aside and tries to impress him to act seriously which the NPC reluctantly agrees to try. Before going into the Mausoleum to retrieve several artifacts, Pandora's Actor requests to be allowed to leave the Treasury to better serve Nazarick with more efficiency. Though hesitant to move a being such as Pandora's Actor to the other floors, he relents and gives him a Ring of Ainz Ooal Gown, ordering him to keep to the 10th Floor. After that, he orders Albedo to give her Ring of Ainz Ooal Gown to Pandora's Actor and follow him to the Mausoleum.

Inside the Mausoleum, Ainz shows Albedo the Avatara, golem statues made into the likeness of each of the Supreme Beings to serve as a memorial to his comrades and to safely store away their divine gear. He tells Albedo once a day a statue of him will be here. Albedo takes it that Ainz means to leave them and begs him to stay with Nazarick forever. Ainz cannot promise her that and tells her that should he be gone leading Nazarick and the other Floor Guardians will fall to her. He tells her his intent to face Shalltear alone, despite her protests, stating it is his responsibility and he does not wish for his children to kill one another. Albedo seeing his determination, demands that he promise her one thing, to come back.

Returning to Shalltear's location, Ainz Ooal Gown escorted by Aura Bella Fiora and Mare Bello Fiore orders them to monitor the area for interlopers. Separating from them he prepares to engage Shalltear.

Meanwhile, Demiurge who has returned to the Great Tomb of Nazarick, meets with Cocytus and Albedo. He to know the current situation. Upon learning that Ainz intends to face Shalltear Bloodfallen alone Demiurge insists that they should not be allowing this. Despite Albedo's confidence that Ainz will be victorious, he criticizes her acceptance of Ainz's decision. Demiurge attempts to leave to assist his master but is stopped by Cocytus who is under orders from Ainz to prevent any interference. Faced with fighting a comrade, Demiurge relents, however, he warns Albedo that should Ainz perish Albedo must step down as Overseer Guardian. Albedo accepts, maintaining her belief that Ainz will return to them.

Approaching Shalltear, the Overlord sees that she is still in her standby mode. Ainz then prepares for battle, buffing his stats with several spells and arming himself with Quick-Change Items. He casts the Super Tier Magic: Fallen Down which causes massive damage, creating in a gigantic crater where the Floor Guardian stood. Shalltear who been in a stationary position goes on the offensive with her Spuit Lance. Being relatively unharmed, Shalltear prepares to attack her former master.

Aiming the tip of the lance at Ainz the former remains unmoved and counters with Triple Maximize Magic: Explosive Land Mine, blowing her away. He continues the assault casting Maximize Magic: Gravity Maelstrom at her flying body. Shalltear manages to stabilize herself and casts Wall of Stone to shield herself from the spell. The attack is followed up with Maximize Magic: Hold of Ribs to restrain her in a skeletal cage but she shrugs it off easily.