Blizzard is preparing what will probably be the final raid instance of Wrath of the Lich King. It will come in the form of an expansion of the Wyrmrest Temple portal complex in the lower area of the zone, next to the Obsidian Sanctum.
The format of this zone is yet unknown, as is it's release date, but this may be the "Sunwell" of WoTLK. We can expect this zone out sometime this summer after Icecrown Citadel has been exhausted by the majority of raiders.
Blizzard had previously said that Icecrown Citadel would be the last raid of "tiered content." Though it is unknown exactly what that means, we can assume that this new instance will not have a full 5-piece tier set, and may be a lower item level bump than we've seen in the progression from Ulduar to Trial of the Crusader to Icecrown.
There is little lore-wise to explain this instance, but we can assume that the storyline here will involve conflicts that will be explored in WoW's upcoming Cataclysm expansion.
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