For those who used to play WC3 this is totally inecessary, but undead invaded Highelfes land to bring Kel'Thuzad back to life, since Arthas killed him when he was a Paladin. Alliance was Dwarfes and Humans. NOTHING MORE! Back to the Undead, or the Scourge. They slaughtered the Highelfes and laid Silvermoon into ruins. Khel'Thuzad was bringed back to life and summoned the apocalyptic demon Archimonde, who destroyed the last of the Highelfes aliies. Those who survived helped Illidan, but he betrayed them. Humans smashed Qeul'Thalas (their new country) and the few survivors got pissed and renamed themselfes Blood Elfes. Its like Sylvanas "Forsaken". They have nothing to do with either humans nor Night Elfes.

Its maybe not exactly correct but its the story as i remember it =P