What is Ulduar in World of Warcraft

Ulduar is a gigantic stronghold, created by the Titans to contain one of the most dangerous creatures alive - the Old God Yogg-Saron. Four keepers (Freya, Thorim, Hodir and Mimiron) were assigned to keep guard and maintain the defenses, overseen by the Designate Loken.

However, Loken would fall under the influence of Yogg-Saron, and began to subvert his orders, leading to Thorim leaving the facility and a feud between Frost and Storm giants to begin.

Slowly the keepers themselves became addled by Yogg-Saron's influences, greatly weakening the holds on Yogg-Saron, who managed to exert his influence outside of Ulduar. Many of the unexplicable phenomena in Northrend would turn out to be his work.

Finally, a band of heroes managed to enter Ulduar, bring the Keepers to their senses and defeat the physical form of Yogg-Saron. No one can say whether he is dead or just too weakened to manifest, but the world is safe from his influence for the time being.