After untold years it was ended by The Blood King, (Trick), after he used his Blood Sage powers to write the future of the Fae. Having grown tired of the senseless violence but realizing that both sides were beyond talking, he used his blood magic to force a truce between the factions. The Blood King split the Fae factions into the Light and Dark Fae clans, and wrote the Blood Laws under which the Fae would coexist.
Shortly after the peace truce was created, it came into peril after members from both clans rebelled against the division and the Blood Laws. The Blood King's daughter, Aife, was one of the rebels. She killed a leader of the Dark clan in one of their uprisings, and the Dark demanded blood in return. To prevent a restarting of the war, The Blood King handed over his daughter to the Dark Fae, despite the fact that she was a member of his own Clan Fin Arvin. She was condemned to death, but instead of being executed she spent centuries imprisoned by a Dark King, during which time she endured torture.
According to Dyson, The Great War had left scars too deep with some and there were those with old grudges they couldn't drop.
"We were fighting ourselves into extinction." – Trick describing The Great War (Blood Lines)
NEEDS TO BE UPDATED WITH SEASON 4 RAINER INFO (AND VEX'S FATHER).