Yep. In Diablo 1, Diablo was in the body of King Leoric's son, Albrecht. After you defeat him, your charectar puts the stone into his forehead, because Diablo couldn't be contained in the stone forever. Eventually Diablo takes control of your hero, who is then The Dark Wanderer' who is the basis for the storyline in Diablo 2. Who the hell knows where the story will go in Diablo 3 though.