Diablo concept art.

It's not currently known whether Diablo will feature as a boss in the game or not given the ending of Diablo II, though it would be a surprise if the player didn't encounter Diablo at some point. Furthermore, Belial is expected to be the main boss of Act II, and Azmodan the main boss of Act III, which paves the way for Diablo to once again feature as the main boss of Act IV.