Angels and Faceless are eternally opposed to each other, like Light and Darkness, which they worship. In the Ancient Era, Angels and Faceless unleashed a bloody war, in which many civilizations perished, including the Empire of Shantir. As a result of this war, there were very few Angels and Faceless left in Ashan.
Therefore, the Angels, in order to strengthen Elrath's position, enlisted the help of humans.
The Faceless, in order to strengthen Malassa's position, helped the elves of Tuidhanny in the war against humans and the other elves. In return, the elves of Tuidhanny turned to Malassa and became dark elves.