Blizzard announces the Reaper of Souls expansion for Diablo III will be released for Windows and OS X on March 25, 2014, and is now available for prepurchase. Here's word from Battle.net:

On March 25, the crusade against death itself charges forth with the launch of Diablo III: Reaper of Souls for Windows and Mac. Prepare for the onslaught today by prepurchasing the expansion digitally on Battle.net.

Whether you plan to immediately don the armor of the new Crusader character or take your current hero directly into Act V, Reaper of Souls will bring you updated loot rewards, all-new class skills and abilities, game-changing features like the Mystic and Adventure Mode, and highly replayable content in the form of Bounties and Nephalem Rifts.

You also have the option to prepurchase the Digital Deluxe Edition, which contains a wealth of in-game goodies: a collection of exclusive helm- and weapon-transmogrification recipes to distinguish your hero among the champions of Sanctuary; a loyal Spectral Hound minion to keep your adventurers company on their epic journeys; three additional character slots to accommodate new heroes; an in-game Treasure Goblin companion pet for World of Warcraft; and an assortment of Crusader-themed Battle.net portraits and Malthael-themed decals for StarCraft II.