Mount Hyjal

From wow.incgamers.com

Sacred Mountain the Night Elves retreated to after forcing the Burning Legion out of Azeroth together with with the Dragons.

In a lake on top of Mount Hyjal, Illidan Stormrage attempted to remake the Well of Eternity with a number of vials taken from the original well. He was interrupted by Malfurion Stormrage and the dragons, but not before he managed to pour three out of the planned five vials into the waters, forever changing that lake into a source of magic, even if not as potent as the original well.

The great dragon Aspects infused an acorn with power and planted it by the side of the sea, growing a gigantic tree over the lake, to cover its magic properties from luring the Burning Legion or other powerful beings back to Azeroth. Illidan was imprisoned for his act, and kept locked up for the next 10,000 years, until the Burning Legion came back to tap the powers of the well.