Date: Released on November 24th 1997

Hellfire is the only official expansion to Diablo, but it wasn't made by Blizzard. It was created by Synergistic Software and published by Sierra. Many people don't like Hellfire because some of its elements don't fit into the Diablo world. There isn't a lot about "Early Hellfire", but we still will try to cover everything we know as well as put the expansion into a better light.

The game implements many features that were planned for classic Diablo, but didn't make it into the release version. For example most of Hellfire's new enemies come straight from the original diabdat.mpq from the numerous unused enemy sprites. Oils for example were a feature of early Diablo versions, but didn't appear in the final game as well
Besides that, Hellfire more than doubles the available character classes with three unique characters that all play very interesting. We've got 50% more dungeons, more quests and a short storyline for the dungeons. Nothing special, but Diablo's storyline wasn't anything to brag about either.

But even if we don't have as much information about Hellfire as we do about Diablo, there are some features that were planned for Hellfire that didn't make it into the final expansion.
The developers only had a couple of months to create this expansion and so they couldn't finish everything they wanted to. There are two quests (check out the quests section) that didn't make it into the final version of the game. A developer at Synergistic confirmed one of the quests that were left unfinished.
Hellfire originally had one additional character class, the monk. With an easter egg you could unlock another class, the barbarian, which was a modified version of the warrior and with the 1.01 patch, the bard was introduced, being a modified version of the rogue. Almost everybody thought of these classes as "Test" or simple easter egg characters, just added to the final game as a bonus for the players. But Synergistic had bigger plans for the bard and the barbarian.
There are two background story texts available on the net for the bard and the barbarian. It is unknown where they come from, maybe they're just fan creations, but if they're real, it shows us that these two classes were originally planned to be implemented like the monk, with a full set of graphics and speech files. The second, and probably most interesting, hint came with the 1.01 patch. Here the EXE is referring to two MPQ files referring to the bard and barbarian, just like the monk.mpq! This is almost a proof that they wanted to give these characters a unique look, but didn't do so in the end, because they ran out of time.

A couple of unused item graphics exist that are even shown on the box of the game, but don't appear on any of the items in the game.
Imagine Hellfire with two more quests, two additional unique looking characters and multiplayer support out of the box (instead of the hacked hellfrui.dll). I think with the amount of time they had, Synergistic did a terrific job. They implemented long lost features of the original Diablo and added new features to the game, increasing the replay value.
But Hellfire has its downsides. It doesn't have bugfixes that were made available with Diablo patches 1.05 and above and there were some gameplay changes that for example made it extremely hard to use a character using a shield versus monsters with ranged magic attacks (Succubi). Hellfire features a lot of new unique items, but many of them (especially rings/amulets) cannot be found (at least in MP...you can find them in SP if you're very lucky) and there are no unique enemies coming with the new enemy classes.
Today there are a number of fan made patches that try to fix a number of these bugs while keeping the gameplay intact without changing a bunch of stuff. There's the Hellfire Fixed mod by LemmingOfGlory, Hellfire 1.02 by Marsh and Hellfire 1.02 V5 by Belix, to name a few. I recommend checking them out!

Since Hellfire's development cycle was super short, there wasn't a lot of promotional screenshot material. Almost everything we've seen so far is either identical with retail or a mockup shot by Synergistic/Sierra.

A couple of facts about the screenshots:

• First we have complete mockup screens, where basically everything is fabricated and not made ingame
• Then we have screenshots that were taken ingame, but altered to make them seem more exciting
• There are also shots that just don't make any sense at all, but judging by the date were clearly made at Synergistic