Before the Sunreavers, there was Aethas Sunreaver, Archmage of the Kirin Tor. Born of Silvermoon, survivor of the Scourge and Burning Legion assaults on Silvermoon and Dalaran, Aethas Sunreaver ushered in a new age of cooperation and worked to restore the alliance between Quel'Thalas and Dalaran, but in the end, he failed to preserve the neutrality of the Kirin Tor and saw himself and his followers either driven out of Dalaran or imprisoned in the Violet Hold ... the very same prison that the fallen Sindorei prince Kael'Thas once languished within, waiting for death at the hands of Garithos and his human soldiers.
Aethas Sunreaver has always had to walk a difficult line, and it has cost him dearly. It was Aethas who cast the deciding vote that led to Dalaran supporting Theramore as Garrosh Hellscream's forces marched against Jaina Proudmoore's city. It was also Aethas, alongside Rhonin, who suggested that Thalen Songweaver assist in the defense of Theramore, only for Thalen to betray the city to the Horde. After the destruction of Theramore and the death of Rhonin, Aethas even supported Jaina's rise to leadership of the Kirin Tor.
In the end, however, it made no difference. That narrow line Sunreaver sought to walk grew ever thinner, until finally it snapped. Now the Sunreavers are bereft of their home, and forced to rely on the very Horde that they were beginning to view skeptically. Here is how the group the Horde will fight for in Patch 5.2 came to be.