World of Warcraft: Cataclysm is the third expansion pack for the massively multiplayer online role-playing game (MMORPG) World of Warcraft, following Wrath of the Lich King. It was officially announced at BlizzCon on August 21, 2009, although dataminers and researchers discovered details before it was officially announced by Blizzard. The expansion was officially released on December 7, 2010.
Shortly after the announcement of the release date, on October 12, 2010, Blizzard implemented patch 4.0.1, which included the overhaul of the game's playing systems. More: en.wikipedia.org.