Hour of Twilight is the third 5-player dungeon added in Patch 4.3. Players must aid Thrall as he travels to Wyrmrest Temple to eventually meet up with the blue, bronze, green, and red dragonflights. Along the way though, he encounters many members of the Twilight Hammer that must be dispatched.

As the bosses drop ilvl 378 loot, this dungeon is more challenging than other heroics and requires ilvl 353 to queue.

A neat part of the dungeon is how Thrall fights besides you and gives your party special buffs.

From the Blizzard Community site:
"With the Dragon Soul retrieved from the past, the Heroes of Azeroth now prepare for the final confrontation with Deathwing. In the frozen wastes of Northrend, the towering spire of Wyrmrest Temple is under the assault of Deathwing's Twilight forces. In order to save Azeroth, Thrall must arrive safely at the temple with the Dragon Soul in hand."