Juiblex (often mistakenly referred to as Jubilex) is a demon lord 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, since zapping a wand of digging from inside a corporeal engulfing monster drops their HP to 1. The engulf attack will almost certainly make you ill, which causes a delayed instadeath, so do apply the horn (or use another method of curing sickness). Messages can be easily missed in the heat of battle, and illness from Juiblex is so likely that it is best just to apply the horn right after killing him NO MATTER WHAT. If you aren't particularly skilled with your weapon, you may have trouble hitting him; however, as long as you can cure sickness again, you can just allow him to re-engulf you, since once inside him, you will be guaranteed to hit him. Don't forget to apply the horn again if you do become re-engulfed.
Juiblex is one of the demon lords who can be summoned and made peaceful, when a Chaotic player sacrifices his own race on an aligned altar. This means he can appear quite early in the dungeon. If this happens to occur, it may be worth attacking him as soon as you are prepared, because he offers a relatively large XP reward for minimal risk.
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 the Juiblex Express up to dungeon level one.
Juiblex presents a difficult problem for extinctionists. He cannot be stoned, so repeated stoning and unstoning will not work (unlike most other unique monsters). He will never leave a corpse, so undead turning also fails. Life-saved re-kills do not count toward the kill counter, and even those take some effort. First, you have to drop an amulet of life saving into his belly while engulfed. Then you have to zap/cast slow monster or speed monster at him to make him put it on. He will not pick up an amulet, but can be generated with one.
In UnNetHack, zapping a wand of digging to escape Juiblex's engulfing will only reduce his HP to half its current value, instead of bringing it down to one.
In Slash'EM Extended, zapping a wand/spell of digging while engulfed by Juiblex will only reduce his HP by 20% of its current value.
Juiblex is an original creation of Dungeons and Dragons. As the encyclopedia says, he is a slime-like being.
Little is known about the Faceless Lord, even the correct
spelling of his name. He does not have a physical form as
we know it, and those who have peered into his realm claim
he is a slime-like creature who swallows other creatures
alive, spits acidic secretions, and causes disease in his
victims which can be almost instantly fatal.