If he carries the Eye (magic resistance), do not attempt to fight him with magic missile, force bolt or finger of death (nor their wand counterparts), which are completely ineffective. Novice players often fall for that error.
Like most covetous monsters, The Dark One should be fought from the upstairs, so that he cannot escape from you when he gets hurt. Preventing him from doing so could be a bit difficult without teleport control and a means of teleportation. A scroll of earth can be very useful in that case. Read it after you get to this level and move one boulder onto the stairs. Another possibility is to use wand of teleportation on him and take his place on stairs.
He will place greater priority on healing than picking up the Eye, so if you can approach him without waking him and hit him hard, e.g. with a reflected magic missile, he will teleport to the upstairs and leave the Eye for you to pick up. His level allows teleport, so you can sneak attack him, grab the Eye of the Aethiopica after he teleports to the upstair, bag it, and teleport near to the upstair to allow you to unload with spells such as magic missile. Bagging the Eye is required because if you don't, he may steal it back and become magic resistant. The simplest way to defeat him is, of course, zapping him with a wand of death or the finger of death spell.
Sleep rays also work well on him. He will be helpless for several turns, you'll avoid the bother of chasing him, and also you prevent him from casting his nasty spells. Make sure to zap him every few turns, to make sure he will not run away.
Better avoid draining him, it will be too slow and unbearable with his spells.
The encyclopedia entry seems to imply that the Dark One is in fact Sauron, the chief antagonist of Lord of the Rings.
... But he ruled rather by force and fear, if they might
avail; and those who perceived his shadow spreading over the
world called him the Dark Lord and named him the Enemy; and
he gathered again under his government all the evil things of
the days of Morgoth that remained on earth or beneath it,
and the Orcs were at his command and multiplied like flies.
Thus the Black Years began ...