The duel was never with the threat of death. If for nothing, Caius had the Hellsingers on standby. He also had Alexia—confused as she might be right now—as his chaperone. No, he was sure Duncan was not insane enough to be planning to kill him.
The Duel was an attempt to knock him down a peg.
Caius still had the right to refuse. No matter the authority Duncan was representing, Caius was still a Noble and Duncan was not the Emperor. He couldn't just order Caius to be arrested or executed because he didn't agree to a Duel.
The worst that could come of Caius saying no, was to create a precedent that Caius—and by extension, the Von Helsing Family—didn't respect Imperial authority.
But Caius saw no reason to refuse. This was the first time in almost a while that he could test his skills against someone who couldn't hurt him. He sparred a lot in the Leopold Coven but the Vampires were always at his mercy even if he didn't enact that authority over them right then.
