This article's content is accurate to the most recent public game version, 0.4.8d.
The Necromancer Lovepet Game Over is an ending which occurs when the player gives in to having sex with the Newbie Necromancer instead of resisting while in the Necro-Yuri Lair.
Eric(a) became so passionate in kissing the necromancer, that when Yoshida tries to pull them away they turn hostile and bite her in the neck, turning her into a zombie almost immediately before return to furious love-making. Now that they've given in to their instinct they become the necromancer's new lover/pet, although the reanimated Yoshida that accompanies them still gives them the stink eye from time to time.