From Halopedia, the Halo wiki
Right hand of Atriox.
Decimus is one of Atriox's most trusted chiefs, though he secretly continues to harbor belief in the divinity of the Forerunners. A prime example of Brute power and rage, Decimus' bloodlust whips troops under his command into a frenzied rampage and his chieftain's hammer can be wielded to unleash a devastating gravity vortex.
As befitting his rank, Decimus has his own personal Brute Warlord lieutenants but he also relishes the chance to make a personal appearance on the field against particularly stubborn enemies.