The Dark Wanderer is the hero who destroyed Diablo in the original Diablo I game, underneath the town of Tristram. After killing [Diablo], the hero saw no choice but to contain the evil within himself, by ramming Diablo's soulstone into his skull. The hero fought a constant battle with Diablo untill Diablo released his brother Baal and set to find the eldest brother Mephisto in Kurast. Mephisto then releases Diablo from the the Dark Wanderer's body, killing the hero but freeing the demon within.
Diablo 1 epilogue on youtube - Here]