"You're holding the power to change the world, Causer. It's right there, in the palm of your hand. Something so much bigger than you or me. And you're going to throw it away?
-Gracius to Timothy Ashford
Gracius and his Frumentarii had been sent to the Royal Wasteland simply to scout and keep tabs on the area, until they got wind of Hades having arrived in the Wasteland. Gracius sent word of this to Caesar, who ordered Gracius to capture Hades. Having another figure from mythology on their side-- Caesar claims to be a Son of Mars-- would help improve the Legion's claim and strength in the eyes of Legionnaires and wastelanders alike. Not wanting to risk his own men in what would be a costly confrontation with a psyker, he instead captured residents of the wasteland and tried to make them do it. He was unsuccessful, until he got his hands on the Outsiders. They were successful in capturing Hades, using a psychic nullifier provided by Gracius, but Gracius attempted to double cross them. As a result Timothy Ashford unleashed Hades, who promptly slaughtered the Frumentarii, and maimed Gracius. The Outsiders unintentionally left Gracius alive, thinking him killed in the chaos, but Kheiro and his Hundred Hands came upon the bodies of the Frumentarii later, and finished Gracius off.
Gracius was very skilled in manipulation and coercion. He convinced the group to help him, first by attempting to seem like an honest, loyal soldier, and when that didn't work completely, he forced them to do it on pain of death. Never raising his voice, and ever presently calm, Gracius always appeared in control of the situation, until the very end, when he released the Outsiders, particularly Ward, were perhaps more dangerous than he'd realized. He lost his cool and blindly ordered his men to attack, which cost them, and eventually him, their lives.
As a praetorian, Gracius would've been a master of unarmed combat, but never got a chance to display his skill. He was a very tall and physically imposing man.