From Halopedia, the Halo wiki
Michael Anthony Claudio Wincott (born January 21, 1958) is a Canadian actor with a distinctively deep, gravelly voice. The younger brother of fellow actor Jeff Wincott, he is best known for his performance in The Crow.
He contributed his voice to Halo 2 as the Prophet of Truth. Michael Wincott was unwilling to continue his role within Halo 3, so he was replaced by Terence Stamp as Truth's voice.