The Dark One occupies the cross-aligned altar of the abandoned temple within his dungeon on the goal level, with the quest artifact and Bell of Opening underneath him.
The Dark One is widely considered to be one of the easiest nemeses to defeat: Many of his spells will be ineffective against a Wizard with magic resistance, e.g. from their starting cloak of magic resistance or Magicbane, their first sacrifice gift. Although wearing The Eye of the Aethiopica confers magic resistance, monsters will not wear amulets of ESP, so you are free to use wands of death or the finger of death spell on him. However, his high monster MR makes him likely to resist other effects such as sleep rays or polymorph beams.
Like most covetous monsters, The Dark One should be fought from the upstairs, but preventing him from getting there first can be a bit difficult without a means of controlled teleporting. 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 teleport and take his place on the upstairs.
If generated at level 16 or higher, the Dark One will be able to summon nasties, so if this is likely have a plan for crowd control or even escape ready.
The Dark One first appears with most other quest nemeses in NetHack 3.1.0. From this version to NetHack 3.4.2, the Dark One did not have a set gender despite being referred to by "his" once in the quest text; this was fixed in NetHack 3.4.3 via commit f11d009.
In NetHack 3.4.3 and earlier versions, including some variants based on those versions, carrying The Eye of the Aethiopica is enough to grant magic resistance; this makes magic missile and the finger of death (or wand of death) completely ineffective against him. The Dark One also tends to place greater priority on healing than picking up the artifact, so a Wizard can approach him stealthily or else teleport directly in line with him, then use a wand or finger of death to dispose of him without a fight. Wizards lacking either option can instead hit him hard (e.g. with a rebounded magic missile spell), forcing him to warp to the upstairs and leaving The Eye of the Aethiopica free for them to take - the Wizard can then deal with the Dark One at their leisure, stashing The Eye in a bag or other container to prevent him from stealing it back.
The encyclopedia entry vaguely implies that the Dark One is an avatar of Sauron, the titular chief antagonist of The Lord of the Rings by J.R.R. Tolkien. The same work reveals Sauron to also be the "Necromancer" of The Hobbit, while The Silmarillion describes him as the chief lieutenant of the first Dark Lord, Morgoth.
Sauron rules the land of Mordor and has the ambition of ruling the whole of Middle-earth; he appears most often as a disembodied, flaming "Eye", though he is briefly seen in a humanoid form in Peter Jackson's film trilogy. Though Tolkien denied that absolute evil could exist, he stated that Sauron came as near to a wholly evil will as was possible.
UnNetHack changes the Dark One's name to Anaraxis the Black; this is a cosmetic change only.
In EvilHack, the Dark One is now an illithid that generates with a robe, and has a 1⁄4 chance of generating with a cornuthaum; he is also given an ancient pseudodragon familiar to match that of starting player Wizards. The Dark One's dungeon also features a pair of mind flayer larva nurseries. The Dark One now has a quest artifact-stealing claw attack, a brain-sucking tentacle attack, and can cast two mage spells per round; he will also wear The Eye of the Aethiopica, conferring its magic resistance and half spell damage upon him.
Overall, this incarnation of the Dark One is possibly among the hardest encountered in any variant of NetHack.
Monster stats by variant
... 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 ...