The Storm Peaks, where Ulduar rests, are full of Titan temples and ruins. The Titans are the god-like creators of Azeroth. In a sense, you could say they did more teraforming than creating, because the world existed but not much of its beauty, or creatures. The Titans, as they had done with many worlds before, left Azeroth after their work was done leaving behind Watchers to safeguard the world. These watchers are: Archaedas, Creteas, Freya, Hodir, Ironaya, Jotun, Mimir, Thorim, Hodir, Lokem, and Tyr. Most of them would normally be found in northrend, guarding over important artifacts. As players questing in the Storm Peaks discover, however, most are nowhere to be found. There is some indication in the quest lines that the watchers are either gone, or dead.
When Ulduar released on the PTR we discovered that many of these Watchers had actually been captured and corrupted and are now bosses in the new raid instance. Included are Freya, Hodir, Mimir, Thorim, and Hodir. Lokem is a boss in the Halls of Lightening 5-man instance that players can defeat, as he is the betrayer of his fellows.
Now, lets look at the other side of the Ulduar Lore: The Old Gods. The Old Gods are horrific creatures that held dominion over Azeroth before the Titans arrived. Although the Titans were content to do their creative thing and leave the Old Gods well enough alone, the Old Gods instead inflicted the Curse of Flesh upon the Titans' earthen creations. The Titans realized that if they were to destroy the Old Gods, that any Cursed beings would be destroyed as well. Instead, the Old Gods were imprisoned below Azeroth, and structures raised to keep them penned in.
As it turns out, Brann Bronzebeard discovered Ulduar to be a part of the prison of Yogg-Saron, the Old God of Death. Inside players encounter an Iron Army, and several bosses including the fallen Watchers who are all under the sway of Yogg-Saron, who has burst free of his shackles. Their mission is to destroy anyone who tries to enter.