From wow.incgamers.com

Darnassus is the home of the Night Elves, and High Priestess Tyrande. It is a city on top of the great tree Teldrassil. Teldrassil was planted using the magical waters of the Well of Eternity in a desperate act to reclaim the Night Elves lost immortality. In the years following the third war, it has become corrupted, as Malfurion predicted.

After the release of the Burning Crusade, there will be a flight path to and from The Exodar and Mages will be able to teleport from Shattrath City once they hit Level 70.

Zombie Invasion Quest

The Zombie Invasion world event that led to the release of Wrath of the Lich King had a quest titled: Darnassus. This quest was offered by Grand Apothecary Putress during phase 5 (October 26th, 2008). The quest below comes from Desperate Research.

Darnassus

Tyrande Whisperwind: What is this? What a boon! They have captured such Light into this artifact, it shines nigh bright as the moon! Our priests will channel this power against our foes, and the Lich King's invasion will surely fail. Thank you, (name). Our world tree will be safe.

What's more, we have sensed that perhaps another world tree grows far to the north, nestled hidden near the Lich King's own domain. Perhaps someday ... we will shepherd both trees and herald a new era of peace.