The Third War (Warcraft 3)
During the Third War, the evil prince Arthas Menethil laid waste to Quel'Thalas, wiping out most of its population and reducing large tracts of the mighty kingdom to ash in his quest to reach the Sunwell. Yet not all who fell before Arthas stayed dead: his courageous archenemy, Ranger-General Sylvanas Windrunner, was raised into undeath to serve the merciless prince as a powerful, tormented banshee.

As the undead armies closed in on the Sunwell, a high elf named Dar'Khan Drathir (who hoped to gain the favor of the Lich King) aided Arthas by lowering the shields surrounding the Sunwell. Dar'Khan's treachery resulted in an explosion that knocked him unconscious and scattered most of the Sunwell's powers.
The wizard Borel (also known as Krasus and the dragon Korialstrasz) sensed the mystical energy being unleashed and succeeded in trapping a portion of it inside an avatar disguised as a young human girl, Anveena. Unaware of Borel's deed, Arthas then used the remaining energies of the Sunwell to reanimate the spirit of Kel'Thuzad in the form of a nightmarish lich.

In the aftermath of the battle, King Anasterian lay dead along with the members of the Convocation of Silvermoon, the high elves' ruling body. Lor'themar Theron, Sylvanas Windrunner's second–in–command, assumed temporary leadership of the high elves, for the true heir to the throne, Prince Kael'thas Sunstrider, was still in Dalaran. The few remaining high elves to survive the Scourge's invasion quickly grew ill and apathetic.
It became clear that the high elves had become addicted to the Sunwell's arcane energies. Being constantly suffused in magic had fundamentally changed their race. Now that the source of their magic was gone, they were suffering acute pangs of withdrawal.