Yin and Yang
From DDwiki
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 beat them. 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, as they are 
Bloodless. In the Alpha they were not; 
Vampire characters or followers of 
Dracul might have some dastardly use for an endless supply of enemies...
If you can manage to kill them, you will be granted the 
Blade of Yin and 
Yang's Sword, which are small items that grant +3 base attack and can be consumed to grant a layer of 
Death Protection.
