Well of Eternity is a 5-man, level 85 instance introduced in patch 4.3. It is the second part of a three-part series of dungeons involving the recovery of the Dragon Soul, preceded by End Time and then followed by Hour of Twilight.
The story involved in this instance is that players will have to travel back in time to the final battle of the War of the Ancients, where they must fight their way to the Well of Eternity and recover the "Dragon Soul" (also called the Demon Soul), the only weapon that can destroy Deathwing, before it is hidden by Malfurion Stormrage.