This article's content is accurate to the most recent public game version, 0.4.8d.
The Zombie Queen Game Over is an ending which occurs when the player triggers two sexual event while in the Necro-Yuri Lair.
Becoming extremely aroused from the Zombification Ring's effect, Eric(a) pulls Yoshida in for a kiss, draining her life and turn her into their first zombie thrall. With new corrupt power Eric(a) turns to, and indeed, converts almost all the necromancers in the lair into their minions. By the time the Yurimancer returns she briefly fought Eric(a), before joining forces on a common goal.
And thus, Zombie Queen Eric(a) was born. Maybe one day she'll be saved or slain by their former Magical Girl friends, but that is a story for later.