The elders of Exo were the Guardians before The 12 guardians now. They set the tweleve Guardians away to live a normal life. The demons were all slaughtered. That's until they found out they didn't kill all. The one that survived reproduced more demons on its own. They were hidding out unsenced and unknown by the elders. When they find out they send The Heart of The Tree (boa) to seek out the Guardians.