If he is not generated elsewhere in a game, e.g. via bones, Juiblex is guaranteed to appear at the center of his swamp. There is a 1/260 chance that a water demon may summon him or Yeenoghu, making it possible for him to appear quite early in the game and obliterate a weak character if they try fountain dipping or quaffing. He can also be summoned by same race sacrifice, and will be generated as peaceful if you are chaotic.
Like other demon lords, Juiblex is covetous and will teleport to meet you. Because of his infravision, he can see you across his swamp, and will typically confront you a few turns after entering the level, as soon as he can get a turn. Dealing with him in a straight fight is tedious, despite his incredibly low speed; on top of his low AC of -7 and the usual troubles with covetous characters, his illness-inducing engulfing attack can and will kill you shortly unless you can cure it with a eucalyptus leaf or a unicorn horn. You will then have to escape his grasp before becoming ill again, which may give him time to teleport away or else re-engulf you.
Thus, the most common strategy uses a preferably blessed unicorn horn, a wand of digging, and a decent weapon of any kind: when he engulfs you, zapping the wand will tear through him and drop his HP to 1, like any non-whirly engulfing monster. Cure the illness as soon as possible, especially once you have successfully disposed of him; if you are having trouble hitting due to his naturally low AC of -7, you can allow him to re-engulf you for a guaranteed hit. If you do so, do not forget to cure your illness again.
As one of the demon lords who can be summoned, Juiblex can be made peaceful if a chaotic player sacrifices his own race on an aligned altar. If this occurs, it may be worth attacking him as soon as you are prepared for the relatively large XP reward, with comparatively minimal risk and only the three required items above.
If you allow Juiblex to engulf you, and then damage him a bit, he'll teleport to the up staircase with you inside of him. This allows easy traversal of his no-teleport swamp lair, especially when coming up from the Sanctum with the Amulet of Yendor. With regular curing of the illness and attention to your HP, 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. Extinctionists looking to add Juiblex to their list will have to drop an amulet of life saving into his belly while engulfed. Then, zap or cast slow monster or speed monster at him to make him put it on. He will not pick up an amulet by normal means, 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.