Blizzard thought it would be best to celebrate the 20th anniversary of the original Diablo game by remaking said game in Diablo 3. The game will land in the game through an anniversary update called “Darkening of Tristram.” This update introduces the graphics, gameplay, and style of the first game to Diablo 3. There will be a 16-level dungeon with four main bosses from Diablo. You will be limited to the 8-degree-of-freedom movement and the old-school filter to invoke the vibe of the original game. The update will be free when it comes out. Those who are in the Diablo 3 Public Test Realm server can play the game as early as next week.
Source: The Verge