From Diablo Wiki
He has travelled across Sanctuary in many forms, last time in the guise of the Dark Wanderer before he was slain by a band of heroes in his own Chaos Sanctuary in Hell.
While his personal character has not been much explored in the games, many of the Diablo novels touch more on the non-combat actions of Diablo.
Regardless of his bedside manners, he will be back, and hungry for human flesh.
Read more about the main protagonist of the Diablo game series on the Diablo article.