Well of Eternity is the second 5-player Heroic dungeon added in Patch 4.3, Hour of Twilight. Players go back in time to assist Illidan and Tyrande in battling the Burning Legion. It is rich in pre-Sundering Night Elf lore, as players get to battle Queen Azshara and are transformed into Night Elves for the duration of the instance.

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:
"Ten-thousand years in the past, the night elven capital city of Zin-Azshari was a magnificent, sparkling metropolis. Situated on the shores of the Well of Eternity, it was here that the aristocratic Highborne first opened a portal into the Twisting Nether and invited the Burning Legion to invade Azeroth. Under the command of the night elven queen, Azshara, they now pour their energies into the portal, preparing it for the terrifying and glorious entrance of the demon god Sargeras."