Cassian is an Illyrian member of the Night Court and part of Rhysand's Inner Circle. He rose to become the General Commander of Rhysand's - the Night Court's - armies. He is considered to be one of the most powerful Illyrians in Prythian history, along with Rhysand and Azriel.

Cassian is tall with black, shoulder-length hair, massive, membranous wings, golden-brown skin, and hazel eyes. He is described to be incredibly handsome however in a more rugged way than his two brothers. 