Hellfire is the mostly-official single-player expansion to the 1996 Blizzard Entertainment classic, Diablo. However, it was not actually developed by Blizzard (who were working on the eventually-cancelled Warcraft Adventures: Lord of the Clans); instead the development duties were outsourced to Sierra On-Line. The expansion adds two additional four-level dungeons, a completely new playable class (the Monk), two unfinished playable classes that can be accessed with a simple config file edit (The Bard and the Barbarian), as well as many new magic items, spells, and other tweaks. Blizzard does not recognize the events of Hellfire as canon or as an official part of the Diablo series ( much like the two Starcraft Mission Packs) and so it has never been redistributed since its original release in the same way as Diablo, and is not included in the Diablo Battle Chest. It can still be found on sites like Amazon, where it is considered a fairly rare collector's item.
Hellfire was developed based on version 1.04 of Diablo, so it still features many bugs that were later corrected in Diablo patches 1.05 - 1.09. Hellfire received only one patch of its own, bringing it from version 1.00 to 1.01.
Hellfire is a separate executable from Diablo and therefore has separate save files and does not integrate with the base game. Character save files can be manually converted from Diablo to Hellfire by renaming their file extension, as described in Hellfire's Readme, but the conversion is not reversible and destroys all items in the character's inventory.
Hellfire adds the Monk to the original playable character classes of the Rogue, the Warrior, and the Sorcerer, bringing the total to four. However, with some easy modification of game files, it is also possible to unlock the Bard and the Barbarian, unfinished but playable classes who share the same sprites as the Rogue and Warrior (respectively) but have different stats and abilities. The Monk is a character with fairly balanced stats whose attacks while bare-fisted or wielding a staff are better than the other classes and has a search skill that can be activated to highlight items on the ground, an ability that would later become part of the default game interface in Diablo II. The Barbarian has the highest strength and vitality attribute possible of any class, and can use certain two-handed weapons in only one hand, but has a maximum magic attribute of 0, preventing him from learning any magic. He has a rage ability that increases his damage for a few seconds up front, then decreases his damage for a corresponding time while cooling off. The Bard is the most well balanced class in the game and can dual-wield any one handed weapons. She does not excel in any one area. She can identify items without using a scroll or going to Deckard Cain.
In addition to the original 16-level dungeon, there are also two additional 4 level dungeons in Hellfire which can be accessed once the player has reached a certain level of the main dungeon. Both of these dungeons have completely new monsters. The Hive is an insect-like area that has a greenish color pallet found on the bridge near the herd of cows. The boss of the area is The Defiler, who, when defeated, gives the player the item used to access the second new dungeon, the Crypt. The Crypt is found in the church graveyard and has lots of undead monsters. The boss of the Crypt is Na-Krul, a lieutenant of Diablo, who is the primary boss of the expansion.
Items and Spells
Hellfire adds several new magic prefixes and suffixes to the pool of magic items, as well as more unique items. The game also adds blacksmith oils which are consumable items that can be applied to a weapon to increase it's effectiveness until the player exits the game (at which time all oil buffs are voided). Some of the new spells added in Hellfire are simply slight variants of existing spells, such as Lightning Wall, but there is also the Search spell that highlights all items on the ground--extremely useful in a darkly lit loot-gathering game. In addition, Apocalypse, the most powerful spell in the game, could be found in spell book form and learned for unlimited usage, as opposed to only being temporarily useable via staves and scrolls like the base game.
Hellfire adds the ability to run in town (movement animation plays at double speed). It also makes Diablo himself noticeably harder and allows the player to play on Nightmare and Hell difficulty in single player. Even though the game is officially single player only, with the modification of game files it is revealed that at some point, the game was going to be playable through Battle.net. This modification allows the game to be played in local multiplayer over LAN or online multiplayer with a tunneling program like Hamachi.