In 3.4.3 and earlier versions, 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. From 3.6.0 on, the Eye must be worn to confer magic resistance, making those strategies viable.
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 a wand of teleportation on him and take his place on stairs.
The Dark One will place greater priority on healing than picking up the Eye, so if playing earlier versions of the game, 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 also 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 unload with spells such as magic missile. The simplest way to defeat him afterward is, of course, zapping him with a wand of death or the finger of death spell; bagging the Eye is required in pre-3.6.0 versions because he may steal it back and become magic resistant.
While he is not guaranteed to be able to summon nasties, if he is generated at level 16 or higher he will be able to do so. Be prepared to escape or strong enough to fight back in such a case. Even when vulnerable, attempting to drain him may prove too slow unless you're willing to accept the risk and try to drag him under level 16. The Dark One boasts a powerful repertoire of spells, so Sleep rays will also work well on him. Applying it every few turns will keep him helpless, saving players the bother of chasing him, and also stops him from casting his nasty 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 ...