Blood Elves

During the Third War, the undead Scourge destroyed the High Elf capital and the source of their power, the Sunwell, along with most of the High Elf population. The remaining High Elves split into two factions - approximately 10% kept their original heritage, but the rest followed Prince Kael'Thas Sunstrider and began calling themselves Sin'dorei (which in their tongue means "children of the blood" - more commonly referred to as "Blood Elves") in homage to their loss. With the leadership of their prince, the Blood Elves continued to defend their homeland from the terror of the Scourge. In their continued struggles on Azeroth, Kael'Thas met the Naga, led by Lady Vashj, and accepted her help in protecting his people. This led to much suspicion from the humans for whom Kael'Thas was stationed under. Grand Marshall Garithos, leader of the remaining Alliance forces in Lordaeron, accused Kael'thas of consorting with vile beings and imprisoned him in the jails of Dalaran. Kael'thas and his lieutenants were eventually rescued by Vashj and her Naga. They then proceeded to flee to Outland to rendezvous with the exiled Illidan Stormrage who with their aid later dethroned the Pit Lord Magtheridon and gained dominion over Outland. Kael'Thas later betrayed Illidan and began seeking power on his own, forming an alliance with the Burning Legion. Cheating his death in Tempest Keep, he later returned to the Isle of Quel'danas and re-ignited the Sunwell, where he attempted to summon Kil'jaedan into Azeroth.

The elves who remained in Eversong Woods have renounced their allegiance to Kael'Thas and are now led by Lor'themar Theron. The Alliance mistrusts the Blood Elves and their lust for a source of magic, so their race has aligned with the Horde for the mutual benefit of reaching Outland.