He moved his guard Knightmares out to fill holes, letting Lelouch get close enough to speak.
Game over, man.
But here's the kicker: Lelouch is a lost prince. 17th in the line of succession. He tells Prince Clovis that he has returned... to change everything.
Lelouch wants answers. Specifically, who killed his mother? He suspects assassination.
With use of the power, Clovis gives up that it was masterminded by Prince Schneizel and Princess Cornelia.
Lelouch kills Clovis. Can't change the world without getting your hands dirty.