At the end of Diablo I the hero defeats [[Diablo]], and takes Diablo's [[Soulstone]] within his own body, intending to contain Diablo there forever. Doesn't work to well for him. 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 the Dark Wanderer and left [[Tristram]]. Immediately after his departure, hordes of demons killed almost everyone there. 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.

After the destruction of Tristram the Dark Wanderer proceeds to the east where he frees [[Baal]] and [[Mephisto]] from their respective prisons before shedding the last of the hero's humanity and becoming fully Diablo again, ceasing to be the Dark Wanderer, and entering hell where Diablo is eventually killed by the hero(es) of Diablo II.