The US release of the game included interviews with some of the programmers for a then yet to be released game called Half-Life. The videos also show some very early beta footage of the game.
Due to Hellfire being published by Sierra, it was NOT included in Diablo Warchest, which includes Diablo, Diablo II, and Diablo II: Lord of Destruction expansion, as well as strategy guide for each.
Hellfire was marketted by Sierra, not Blizzard, though it was an authorized add-on, unlike a flood of cheat files and editors that was released around that time.
Hellfire has two hidden characters you can play as, the Barbarian and the Bard. The barbarian looks exactly like the warrior, but can wield axes as fast as swords, hit multiple enemies with them, and has 0 max magic. The Bard looks exactly like the Rogue, can wield two one handed weapons for more damage. There are also two hidden quests, and different color palettes for the two new areas. To access these, create a command.txt file in the Hellfire directory containing the line
Or you can only include the hidden features you want. Also including the Cow Quest changes Na'Krul's speech with hillarious effects.