The Oathbound are people loyal to the leaders of Erdas.
The Oathbound wear black uniforms with bronze collars and vambraces. The ones with spirit animals seem to be leaders of regiments.
In the Books
The Oathbound relentlessly pursue the Four Fallen throughout the book until a battle near the end, where Rollan creates a tunnel using the Heart of the Land (object), where the Oathbound cannot follow them.