Diablo: Hellfire added the Monk as a playable class to the original Diablo.
Hellfire was an expansion developed by Synergistic Software and released in 1997. Although Sierra Entertainment published it under license from Blizzard Entertainment, Blizzard did not develop the expansion itself. The Monk gave players a new close-combat specialist focused on staves, fists, and fast melee attacks.
The Monk’s presence is one reason Hellfire is easy to distinguish from the original Diablo. The expansion also introduced new quests, items, monsters, dungeons, and a non-player character named Lester. Some of its content was not officially integrated into later Diablo continuity.
The Monk should not be confused with Diablo II’s Paladin or Diablo III’s Monk. Diablo III’s Monk is a different class with a broader skill system and a distinct story background, even though both are martial-arts-oriented heroes.