This demon lord's lair is located between the 4th and 7th levels of Gehennom (and therefore between DL 29 and DL 36).
Juiblex' swamp is eligible to leave bones files. The entire level is no-teleport.
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Legend
This core of the level is surrounded by a similar mixture of land and water (though with two guaranteed islands to ensure at least one place for each stair to go; the upstair is to the right of the above area, and the downstair to the left).
Juiblex is at the exact centre of the map; three lemures occupy the large island on the centre-right of the map above, and all but (a random) one of the marked fountains are actually giant mimics masquerading as fountains. The mapped area also contains six random P, five random b, five random j, five random F, two random m, and two jellyfish; two sleeping gas traps, two anti-magic traps, and two magic traps; six random potions, three random food items, one boulder and finally two random gems (or piles of gems) at the bottom of the pools on the lemures' island.