There are initially 10 black Cait Siths and 10 white Cait Siths from a different dimension associated with Voidwatch. At one point in the story, the black Cait Siths combined to form a single entity. It is this entity that is a summonable Avatar available to players who complete the Wings of the Goddess expansion as well as the quest "Champion of the Dawn" or "The Dawn Also Rises".