Ancient of corruption
Ancients of corruption are a possible inhabitant of an ancient-nupperibo-hellish seal. They are notable for their many sliming and disease-causing attacks, their noticeable physical damage, and the wide variety of other status ailments they can inflict.
Cursed unicorn horn
The ancient's butt attack has the same effect as applying a cursed unicorn horn.
An ancient of corruption inhales heartiness and exhales burning corruption.
"Slime bubbles up from under your skin."
The inhalation affects one valid target within 4 squares of the ancient at random. A valid target is one that lacks either fire or sickness resistance, and is not a green slime or slime remnant.
The attack deals 1d6 damage per 3 levels of the ancient (so 5d6 for the typical ancient), doubled vs. wooden targets or targets that inherently resist cold damage and that are not fire resistant. If this damage kills the target, it becomes a slime remnant. In addition, if the player character was the target they become deathly ill, and if the target was a monster their max HP is reduced by a like amount.
"The ancient of corruption breathes out a spout of slime!"
The ancient releases an 8x8 explosion of acidic slime which deals 1d12 damage per 3 levels of the ancient (so 5d12 for the typical ancient). Both acid resistance and slime resistance protect against the damage, but unless the target is slime resistant they are infected by sliming.
Slime remnant monsters lose many of their attacks, in return for a few acid-based melee attacks.
dNetHack ancients are very loosely based on D&D Ancient Baatorians. Only two examples of ancient baatorians were actually encountered during the period in which they were introduced: a nupperibo with constitution-draining tentacles (stated to be a metamorphosing, larval baatorian) and a shapeless thing that inhales light/reflectivity and life and exhales darkness. There were also said to be spined creatures frozen deep under the ice in Cania. A later source claimed that Zargon was another ancient baatorian, but Zargon originated over a decade before the later idea of ancient baatorians (and over two decades before the source tying them together) and is obviously unrelated. That said, the ancient of corruption is based on Zargon's abilities.