Yin and Yang
The Yin Yang subdungeon contains two dangerous enemies, the Eternal Yin and the Immortal Yang. You must slay both two in two consecutive strikes in order to be victories. If you kill one but take more than one attack to slay the other, then the original will merely reappear. Since these are non-XP giving enemies, there is very little way to make use of this eternal respawning, although they are not bloodless, so Vampire characters or followers of Dracul might have some dastardly use for an endless supply of enemies.