Ancient of the burning wastes
Ancients of the burning wastes are a possible inhabitant of an ancient-nupperibo-hellish seal. They are notable for their very dangerous 2-square-reach melee attack, difficult-to-resist breath attack, constant re-location, noticeable physical damage, and ability to relocate the character.
2-square reach attack: mummy rot
When used against other monsters, this attack deals heavy damage.
When used against the player character, the character begins rotting to dust. While the first hit merely begins the process, subsequent hits reduce Constitution or Charisma by 1 to 6 points. If both scores are 3 or less after an instance of this damage, the character dies.
The ancient only uses two spells.
This spell deals normal spellcasting damage (5d5 or 5 - 25, for the typical ancient of the burning wastes) to the target. This damage can be resisted both by half physical damage and half magical damage.
The target is thrown 8 squares in a random direction.
An ancient of the burning wastes inhales moisture and exhales sweltering heat.
"Bloody mist rises from your skin."
The inhalation affects one valid target within 4 squares of the ancient at random. A valid target is one that is either not "anhydrous" or that has blood.
The attack deals 1d5 damage per 3 levels of the ancient (so 5d5 for the typical ancient). If the target has blood and doesn't resist level drain, the attack also drains one level.
Exhalation: sweltering heat
"Heat shimmers surround the ancient of the burning wastes!"
All valid targets within 8 squares of the ancient are damaged. Valid targets are those that lack fire resistance or that are non-living. A target that is both non-living and lacks fire resistance takes double damage. The attack deals 1d5 damage per 3 levels of the ancient (so 5d5 for the typical ancient).
Finally, if the player character is within 8 squares of the ancient, the ancient relocates within reach of the character.
Due to its constant relocation, it is difficult to confront an ancient of the burning wastes with melee attacks. Strong ranged attacks may be used, though many spells and wands are blocked by the ancient's reflection. If the player finds it difficult to confront the ancient, they would be well-advised to use an escape item to retreat and obtain sustain ability to avoid being killed by mummy rot.
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.