Atriox is basically the Arbiter of the Brutes. However unlike the Arbiter Atriox and his clan were sent into suicide missions before being banished.
The Arbiter was immensely successful during the war, his success gained too much popularity for the heirarchs to ignore and thus looked for a way to have him removed to prevent the elites from gaining a leader. Becoming the arbiter he voluntarily participated in suicide missions in the hope that he wouldnt survive.
Atriox fought that way from the beginning and having survived was sentenced to death. His fame and strength gained him respect allowing him to recruit from the covenants forces as he rebelled, the Covenants inability to affect the Banished during the war just shows how powerful they really are.
The innies to humanity are significantly weaker than the UNSC, however the Banished are basically on par. That goes to show just how strong the force actually is even when they dont have the same empirical organization and supply as the Covenant.