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Blizzard has announced the fifth expansion for its flagship World of Warcraft (WoW) online game.

Called the Warlords of Draenor it adds a new world to the game, lets players build settlements and instantly boost a character to high level.

The announcement comes as WoW player numbers hit a five-year low and Blizzard starts to group players on a smaller number of servers.

Blizzard has also unveiled plans for a new game called Heroes of the Storm.

Details about Warlords of Draenor were unveiled at the annual Blizzcon gaming convention. The expansion involves a time-travelling storyline that will see players re-enact some of the events from the series of Warcraft strategy games that predated WoW.

The expansion is set on the world of Draenor and gives players a series of new zones, dungeons and instances to play on. Warlords lets characters play at level 100, introduces player settlements called garrisons and updates the look of some of the game's different races.

Blizzard said buyers of Warlords would be able to boost one character to level 90 so they could instantly go adventuring in the world of the expansion. No release date for Warlords has yet been given.

The news comes soon after Blizzard revealed that subscriber numbers for WoW had dropped to about 7.7 million. The last time they were this low was in 2007 and they are significantly down on the 2010 high of 12 million.

To manage this decline, Blizzard has introduced a system that lets people play across servers to ensure the game's economy, Player-versus-Player areas and arena battle systems have enough people participating to keep them functioning.

Writing on the Rock Paper Shotgun game news site, Nathan Grayson described Warlords of Draenor as a "decent expansion" but said the decision to re-visit Warcraft's past was "puzzling".

"This is maintenance, in a sense," he wrote, external. "Something to keep existing players interested and happy. And who knows? Maybe the focus that a smaller audience allows will be good for WoW in the long run."

Blizzcon also saw the announcement of a new game called Heroes of the Storm. This is an arena-based hero brawling game that lets people take control of key characters from almost every game Blizzard makes including Starcraft and Diablo. The heroes are united in teams and fight rivals for dominance on one of a series of different maps.

A beta test of Heroes is currently under way and Blizzard said the finished game would be released "soonish".