Diablo is an incredibly powerful demon and the primary antagonist of the Diablo video game series.
Non Disney Villains Tournament
While not shown, Diablo is presumably the source of possessing Pitch Black's daughter, (Leah in this universe), through the will of the Mysterious Stranger. Whether he is supposed to make a real appearance in the tournament, it is currently unknown.
Video Game Villains War
Diablo finally achieves one of his dreams and takes over heaven itself. He does not, however, anticipate the arrival of Kefka Palazzo.