The draenai Elders (Drake and Barack) decided to loot the AH of Stormwind creating a Civil War within the Alliance. Impressed by their behavior, the Naga decided to recruit them in the New Faction: Scourge. The only condition: total submission to the Naga and they would get full support from the Naga. The slightest mistake and the Naga would sacrifice them for the Old Gods.
After gathering enough Arthas Tears from Western Plaguelands, the alchemist fire mage troll Veinmetal decided to create a great potion of Resurection that would bring back to life Arthas. The Arthas tears mix with the ashes of Death Wing put on the Throne of Undercity , with a specific Ritual of the Council of Banshees, Arthas the undead Death Knight came back. (he could no longer be human like he once was)
Veinmetal feared that bounding of the Naga with the Draenai could bring a great danger to the horde. And since there was no question to ally with Alliance, Veinmetal's plan was to help Arthas to become the Leader of the Scourge once again, to contain this New Danger.