End Time is a dungeon added in Patch 4.3, Hour of Twilight. Players aid Nozdormu in righting the past and observing darker versions of history. Select faction leaders are corrupted and residing in destroyed dragonshrines--you will encounter two of the four in each dungeon before facing the final boss, Murozond.

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

From the Blizzard Community site:
"One of an infinite number of potential outcomes, this timeway depicts the desolate future of Azeroth should Deathwing fail to be stopped. Nozdormu has identified a powerful anomaly that exists in this moment and bars access to the past - and hope of retrieving the Demon Soul. A powerful creature from out of time living alone amist time-twisted echos of the past."