The Dark Wanderer is a hero in Diablo I and an NPC in Diablo II. How he appears in Diablo III is still unknown.

Diablo 1 (and later)

At the end of Diablo I, the Dark Wanderer defeated Diablo and sealed the soul stone of Diablo in his own body, intending to suppress the power of Diablo forever, but had a negative impact on him. According to Cain's diary, the soul of Diablo took two days to occupy and control him. At this time, he became a dark wanderer and left Tristram. After he left, groups of demons killed almost everyone there. This means that the Dark Wanderer is directly responsible for the massacre, although it is only a small matter in his time.

Diablo 2

After Tristram was destroyed, the Dark Wanderers came to the Eastern World and released them from the respective prisons of Barr and Mephisto. Later, he lost the last bit of humanity of the hero and became a thorough Diablo God, no longer a dark wanderer. In the end, Diablo was defeated by the hero played by the player in Diablo II.

Diablo 3

On the official website of Diablo 3, Kane mentioned the Dark Wanderer in his diary, but there is no other information (concept map, screenshot, demonstration, etc.). He may be mentioned by NPC in the game. This character should not appear as an entity (speaking) in Diablo 3, even as the incarnation of Diablo.

Speculation

Many players hope that the Dark Wanderer is still alive. To be precise, this warrior should have the value of existence, although history can be explained in other ways, and Blizzard has never made an official explanation. Players believe that just like thieves become blood black and mages become summoners, every hero will fall under the corruption of Diablo.