When a research lab in Siberia known as the Tower of Babylon goes mysteriously dark, members of both Schicksal and Anti-Entropy converge on the site to investigate the cause. While their suspicions of a Honkai presence are correct, they discover an even greater threat: Sirin, the 2nd Herrscher, is on a quest of vengeance that threatens the safety of the whole world. Unable to face Sirin alone, Schicksal and Anti-Entropy set aside their differences to forge an uneasy alliance against her -- but even their joined forces may prove to not be enough.