Ixoth, D, is the Knight quest nemesis. He is a red dragon that guards the Bell of Opening and the Knight's quest artifact, the Magic Mirror of Merlin. Strangely, the default tileset gives him an identical appearance to Nalzok and the Minion of Huhetotl.
Approaching Ixoth can be difficult due to the layout and darkness of his cave; unless you have stealth, he will likely become aware of your presence and teleport next to you while you are navigating the southern bend. As with all covetous monsters, it may be more ideal to engrave Elbereth or drop a scroll of scare monster on the stairs and awaken him from there.
In practice, Ixoth will not use his fire breath unless he has teleported to the upstair to heal, since his covetous behavior will put him in melee range. The magic resistance from the Magic Mirror of Merlin will prevent a wand of polymorph or death from working on him, but not the wand of sleep. Magic resistance is recommended to protect your items from curse items and destroy armor.
If you are mounted and are at least skilled in using the lance, you can possibly use your speed to joust Ixoth to death, especially if you can lure him out while remaining on the upstair. Note that while Ixoth will be unable to attack on most turns after being stunned from the joust, it is more likely than not that there will be one or two turns where Ixoth recovers and attacks you before you can joust him. Ixoth could use this turn to teleport to the upstairs or attempt to destroy your armor.
Ixoth shares his encyclopedia entry with all other dragons.
In the West the dragon was the natural enemy of man. Although
preferring to live in bleak and desolate regions, whenever it
was seen among men it left in its wake a trail of destruction
and disease. Yet any attempt to slay this beast was a perilous
undertaking. For the dragon's assailant had to contend
not only with clouds of sulphurous fumes pouring from its fire
breathing nostrils, but also with the thrashings of its tail,
the most deadly part of its serpent-like body.
"One whom the dragons will speak with," he said, "that is a
dragonlord, or at least that is the center of the matter. It's
not a trick of mastering the dragons, as most people think.
Dragons have no masters. The question is always the same, with
a dragon: will he talk to you or will he eat you? If you can
count upon his doing the former, and not doing the latter, why
then you're a dragonlord."