The Dark Wanderer was originally the hero of the Diablo I game, and becomes a NPC in Diablo II. His presence in the Diablo III game is unknown.
Contents
In Diablo I (and the aftermath thereof)
As Diablo I draws to a conclusion, the hero defeats Diablo, and takes Diablo's Soulstone within his own body, intending to contain Diablo there forever- which had potentially unforseen consequences. According to Deckard Cain's Journal it takes about 2 days for the spirit of Diablo to corrupt and posses him. At which point, he became fully possesed and left Tristram. Immediately after his departure, hordes of demons either killed or possessed the remaining denizens with the exception of Deckard Cain whom was only captured. It is implied that the Dark Wanderer is directly responsible for that carnage, although it may have just been a side-effect of his presence.
In Diablo II
The Dark Wanderer (left) and ArchAngel Tyrael (right) do battle during a Diablo II Cinematic.
The Diablo II wiki entry for the Dark Wanderer may be found here.
In Diablo III
The Dark Wanderer is mentioned in Cain's Journal on the Official Diablo 3 Website, but has yet to appear in any other fashion (concept art, screen shots, trailers etc. etc.)
He may be mentioned by NPC's throughout gameplay. It is not likely this NPC or character will return in the flesh (so to speak) during the events of Diablo III, even if different incarnation Diablo does.
Speculation
Many fans believe the Dark Wanderer to be, specifically, the warrior class due to gender and skin tone, although the lore can be interpreted other ways and Blizzard has made no official comment on the topic. Fans who believe this usually also subscribe to the theory that the rogue became Blood Raven and the Sorcerer became The Summoner, each hero succumbing to madness due to Diablo's corruption in their own way.