The Lorcatha are a clan that Diath is linked to.  When Strix met "Ubtao", he told her that the Lorcatha clan is linked to angels. The Skizziks are linked to devils. Some sort of deal kept the two families in a cease fire/non-interference pact. One of the terms of this pact is that members of the two clans can't have anything to do with each other or the multiverse might be destroyed. Also, the Lorcatha had to give up their souls... all except one. Diath's soul is that "ancient soul". 
Later, Izek explained that the Lorcatha were a human clan who tried to destroy the Skizziks. He spoke of the Ashton Concordance, a magical agreement that dictated that Lorcathas and Skizziks could not be together in any kind of alliance.