Which studio developed the original Diablo expansion Hellfire?

Synergistic Software developed the original Diablo expansion Hellfire.

Hellfire was released in 1997 as an expansion to Diablo. Although it was published by Sierra On-Line rather than Blizzard, Synergistic Software handled development. The expansion added a new dungeon beneath the Tristram monastery, additional quests, items, enemies, spells, and the Monk character class.

Hellfire’s relationship with the main Diablo series can cause confusion because Blizzard was the creator and owner of the Diablo property. Blizzard North developed the original Diablo, but Synergistic Software developed Hellfire. Blizzard later treated some of Hellfire’s additions as separate from the core series continuity.

The expansion was made available for the Windows version of Diablo. Its Monk class uses distinctive unarmed combat techniques, while the expansion also contains hidden or optional content. Hellfire is therefore historically important as the first Diablo expansion, even though it was not developed internally by Blizzard North.