Hellfire is the only official expansion to Diablo. It was made by Synergistic Software and was published by Sierra. Many people don't like Hellfire as they think its additions don't fit into the Diablo world. Although there isn't as much information about "Early Hellfire" I'd like to put Hellfire in a better light. The game implements many features that should've already been in classic Diablo, but didn't make it. For example many enemies, which graphics can still be found in the Diablo retail MPQ. Hellfire makes these monsters and other things available to the player. Oils for example were a feature of early Diablo versions, but didn't appear in the final game. Besides that, Hellfire doubles the available Classes with three unique characters that all play very interesting. We've got 50% more dungeons, more quests and a nice storyline for one of the dungeons.
But although we don't have as much information about Hellfire as we do about Diablo, there are some interesting facts about the development of Hellfire. There are some features that were planned for Hellfire that didn't make it into the final game. 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 missing.
Hellfire originally had one additional character class, the monk. With a trick 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" 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, but it might be a hint, 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.