In the run-up to the release of Diablo II, a partnership between Blizzard and Wizards of the Coast was begun. This partnership yielded a number of D&D products based off the Diablo universe, most of which are simply pen-and-paper translations of the Diablo computer games.
Diablo II: The Awakening
The first of the Diablo D&D product line, this book was published in the year 2000 as merchandising attached to the Diablo II computer game. The book's rules are based on the 2nd edition Advanced Dungeons and Dragons rules, and present adaptations of Diablo II's five core character classes (Barbarian, Paladin, Necromancer, Amazon and Sorceress). The book also features a large menagerie of monsters (27 base monster types inspired by both the Diablo I and Diablo II beastiaries, with each monster type having multiple [3+] "pallette-swapped" incarnations that grow increasingly more powerful [sort of like in the computer game]), an expansive treasure table, which includes base items and a number of prefixes/suffixes which provide various special item attributes (again, like the computer game), and a list of spells that is inspired by both Diablo I and Diablo II spells/skills.
Because Diablo II was likely still in development and not made available to the writers of the D&D book, the adventures included with the book are basically a D&D translation of the quests and adventures of the Diablo I computer game, based out of the town of Tristram.
Diablo Fast Play Boxed Set
Making use of the Diablo license, Wizards of the Coast produced this D&D set as part of its "Fast Play" line, which was designed to be simple enough to bring new players into D&D. The interpretation of Diablo mechanics (including the use of an MP system, rather than D&D's traditional "spell memorization" system) was somewhat more faithful in this stripped-down tabletop game, but the stock heroes provided for use by players included only the Paladin, Barbarian, Amazon and Sorceress (skipping out on the Necromancer entirely).
The adventures included in this product were original, written by D&D heavyweights Jeff Grubb and Bill Slavicsek. They revolved around the town of Waystruck, and the quest to stop an overlord servant of Andariel named the Slayer from claiming the Butcher's Cleaver and beginning a new reign of terror.
Diablo II: Diablerie
This was the first Diablo product published for the D&D 3rd edition rules set, and much of the content was devoted to re-presenting "Awakening"'s material in a 3rd edition milieu (though the Monsters section added several Diablo II monster types that were not present in the original "Awakening" book). The skill sets in this book were the most faithful representation yet of the Diablo II skill system, even to the point of some higher-tiered skills having skill prerequisites (like in Diablo II's skill trees).
The book did not include an adventure arc like "Awakening" or the Fast Play set. Instead it included a single dungeon, Morgen Keep, an abandoned fortification in the wilderness. Morgen Keep has three layers: the ground floor (which includes a surrounding wilderness area), the catacombs, and then a cave system beneath the catacombs. The objective for players entering Morgen Keep is to retrieve Siegehammer -- a magical weapon that is harmful to demons -- and defeat Crushskull, the demon who has been sent to the keep in order to guard the weapon and keep it out of demon-slaying heroes' hands.
Diablo II: To Hell and Back
The final book published as part of the Diablo II D&D line, this book re-presented much of "Diablerie"'s material, but also presented the first four acts from the Diablo II computer game as D&D adventures.
Web Accessory: The Secret Cow Level
As a bit of humor, Mike Selinker (one of the original designers of the original "Awakening" book) designed a "secret cow level" module for players of the Diablo II D&D game. This adaptation takes players into "Moo Moo Farms," where they are pitted in battle against a herd of Hell Bovines and the Cow King. The module is available for download at the Wizards of the Coast website.