Hellfire is the only official expansion for Diablo, developed by Synergistic Software and published by Sierra. It adds a monk class, jog toggle (run faster in town), new items like oils, runes and new spells, shrines, enemies, and 2 new dungeons (Hive and Crypt).

The new dungeons are optional so you may still play the main game and avoid the Hive/Crypt completely.