There's a six-page retrospective feature up on Co-Optimus that covers the history surrounding Blizzard's first two Diablo titles, as well as their Hellfire and Lord of Destruction expansion packs. And while it's nice to see that they include Hellfire in the discussion, it's somewhat surprising that they completely overlook Unearthed Arsenal:
Did you even remember that Diablo had an expansion? Blizzard seems to have forgotten as well, since it's not on their list of legacy games. Hellfire was produced by Sierra, not Blizzard, which is likely the reason for this exclusion. Nontheless, it is an authorized expansion, and a fascinating one at that.
The most obvious addition to Hellfire is another class for players to choose from: the Monk. The Monk, as you might expect, excels at unarmed combat, and is the master of the staff. Monks have the ability to hit three targets with one attack, which is ideal for clearing out clumps of bad guys. To compensate for their melee offense prowess, the Monk gets less benefit than the other classes when wearing heavy armor, and they have far less max hit points than the other melee-centric class, the Warrior.
Hellfire added new levels to the game as a lengthy side quest. The Demon Crypt and Festering Nest are the two halves of the additional Hellfire content. New monsters to complement these nifty new tilesets are included. To compensate for the extra time players have to level up in the expansion, Diablo has been buffed up considerably, making him even tougher to take down.
Hellfire made some tweaks to the user interface that would make their way into the official sequel. One of the best is the ability to run in town, instead of nonchalantly walk around. There is a new spell called Search, which lights up hard to locate items and gold that have fallen on the ground. Another spell lets you warp to the nearest staircase, to assist in exploring the dungeon. New items, shrines, and even more spells, including books of extremely powerful Apocalypse and Nova, round out the additions in the expansion. Sadly, Hellfire was designed primarily as a single player experience, though a few of the interface tweaks made their way into co-op.