Irvine-based Blizzard Entertainment reported Tuesday that it has begun closed beta testing for Diablo III, the sequel to its Diablo role playing game series. Blizzard did not say how many users will be added to the beta. The firm said the closed beta is an "important milestone" on the way to final development of the new title. Blizzard is known for its massively multiplayer online games, including World of Warcraft, StarCraft, and Diablo.