Juiblex

Juiblex, &, is one of the demon princes whose lair is a swamp. He can engulf and make ill any character he meets and will spit acid venom.

Like other demon lords, Juiblex can and will teleport to meet you. Dealing with him is quite simple, you only need a unicorn horn, a wand of digging, and a weapon of some kind. When he engulfs you, zap him with the wand, then hit him once and he will die. This is because zapping a wand of digging from inside a monster causes severe damage to that monster; their HP drops to one. Unlike his fellow demon princes, Juiblex is immune to stoning.

If you allow Juiblex to engulf you, and then damage him a bit, he'll teleport (with you inside of him) to the up staircase. This allows easy traversal of his no-teleport swamp lair, especially when coming up from the Sanctum with the Amulet of Yendor. It may be possible to continue this process indefinitely.

Juiblex presents a difficult problem for extinctionists. He cannot be stoned, so repeated stoning and unstoning (which works with most other unique monsters) will not work. He will not wear an amulet of life saving. He will never leave a corpse, so undead turning also fails. Juiblex is the only denizen of the dungeon that can be killed precisely once.