Diablo is one of Blizzard's tentpole franchises, part of a Holy Trinity of PC gaming that includes the universes of StarCraft and Warcraft. It's reverence is well-deserved, as Blizzard has done a fantastic job with all three games.
Old-time fans of the series may remember, though, that in 1997 a Diablo product was released that had almost nothing to do with Blizzard whatsoever.
That product was the expansion Hellfire, which bizarrely was neither developer nor published by Blizzard (though it was of course authorised...we're not talking a bootleg expansion here, people).
The expansion was the work of Synergistic Software, a studio which had been founded all the way back in 1979 and which had survived just fine before being bought by Sierra in 1996. It was Sierra who published Hellfire, and who then a year later shut Synergistic down. Good job, guys!
That closure had nothing to do with Hellfire's quality; it was more to do with Sierra's shoddy performance in the modern era, which saw the former adventure game powerhouse (and publishers of the original Half-Life!) fail to cope with the changing PC landscape of the late 1990s.
Coming out a year after the original Diablo, and featuring a concurrent storyline, Hellfire introduced the Monk, Barbarian and Bard as classes (though the latter two were only available after a small hack), and also some new quests. While it "plugged in" to the main game, opening up new dungeons within Diablo's existing world, its multiplayer modes were not and are not compatible with Blizzard's Battle.net service.
Nor are its events necessarily considered canon, as subsequent games make almost no mention of what takes place in Hellfire. It does live on in some way, though, as various enemies, artistic flourishes and even some gameplay tweaks (like the ability to run) were taken up by Blizzard in Diablo II.