Who actually made Diablo's expansion, Hellfire?

Hellfire (1997), the only expansion the first Diablo ever received, was not made by Blizzard North. It was developed by Synergistic Software, a division of Sierra On-Line, and published by Sierra. The assignment came from above: the two companies then shared a corporate parent, and the parent wanted an expansion while Blizzard North moved on to the sequel.

The paper trail is unusually frank. Polygon's 2018 excerpt of David Craddock's Stay Awhile and Listen: Book II records Blizzard VP Pat Wyatt: "When we were more or less forced to do the Hellfire expansion, it was over our protests, kicking and screaming all the way." Blizzard North's conditions for allowing it, executive-producer credit and final approval, were, in Brevik's word, "summarily ignored." Its hard restrictions held, though: no Battle.net, no multiplayer, and no content reserved for Diablo II. The Barbarian class was vetoed; the one new class Synergistic was permitted was the Monk.

The story ends in reconciliation: in June 2019, Blizzard itself re-released Hellfire as a free add-on to Diablo on GOG.com, officially adopting the expansion it never wanted, twenty-two years later.