Here's a strange little detail history kept: the expansion for the original Diablo, the one that gave the series its first Monk, wasn't made by Blizzard.
It was called Hellfire, it came out in 1997, and it was built by a studio called Synergistic Software. Blizzard didn't develop it and didn't publish it. Today you'll find it bundled free with Diablo on GOG, sitting next to a mode that lets the 1996 original run at 20 frames per second, as it did back then. Old games keep their odd paperwork.
How we know
The plainest statement comes from GOG's own store page. The Diablo listing on GOG says Hellfire was "originally developed by Synergistic Software and released in 1997," and that it "was not an official Blizzard product." The same page says Blizzard has given the OK for it to be included as a free add-on with Diablo purchases, and that Hellfire can be reached by launching the original game.
Blizzard says much the same in its own words. Its announcement of Diablo's arrival on GOG repeats that Hellfire "was not an official Blizzard product," confirms the free add-on, and politely notes that support questions about Hellfire go to GOG rather than Blizzard. That's about as clear a "not ours, but you can have it" as a publisher ever writes.
The backstory
So who was Synergistic? According to Kotaku's 2012 look back at the expansion, it was a studio founded in 1979 that Sierra acquired in 1996. Sierra published Hellfire, Kotaku says, and Blizzard had no hand in developing or publishing it, although the expansion was officially authorised.
What did players get? Kotaku lists the Monk as a new playable class, with a Barbarian and a Bard that were only available after a small hack. There were new quests and dungeons that hooked into Diablo's existing world, with a story running alongside the main game. PC Gamer's report on Hellfire's GOG arrival fills in the villain: the demon Na-Krul, let loose on Tristram by a sorcerer who then traps the player in its lair to deal with the problem. PC Gamer's verdict on that plot point is hard to improve on: nobody in Tristram cleans up their own messes.
The ending for Synergistic wasn't a happy one. Kotaku reports that Sierra shut the studio down about a year after Hellfire, and puts that down to Sierra struggling with the changing PC landscape of the late 1990s rather than to the expansion itself. Hellfire's story also never really made it into the canon; Kotaku notes the later games largely ignore its events, and PC Gamer calls it Diablo's "non-canonical expansion." Still, Kotaku points to bits that did carry over into Diablo II, including some enemies and the ability to run.
Why it's worth knowing
Part of the charm is what it says about how a 1990s expansion could come together: another studio, another publisher, a blessing from the original's makers and a story that never quite joined the main one. Hellfire is that arrangement preserved in amber, right down to the separate support desk.
The other part is how carefully it's been kept. GOG's page describes two ways to play Diablo itself: a classic mode for "the authentic Diablo experience," with "period-appropriate 20 FPS SVGA graphics" and matchmaking through the classic Battle.net, and an updated version tuned for modern PCs, with Windows 10 compatibility and bug fixes. Blizzard's post adds that the classic version is "completely unchanged," while the enhanced one gets audio fixes and upscaling but can't connect to Battle.net. PC Gamer reported in June 2019 that Hellfire itself had been updated with high-resolution support and compatibility tweaks.
So the expansion Blizzard didn't make is now sold by a third company, supported by that company, and patched to run on machines nobody had in 1997. The Monk got the last laugh, really: a guest in someone else's dungeon, still there nearly three decades on.
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