Alcatraz was part of a Force Recon Marine Unit coming to extract Dr. Nathan Gould via submarine, but their insertion goes hay-wire as the Ceph attack the submarine, leaving Alcatraz as the apparent sole survivor

you are Alcatraz

Prophet finds you half-dead in semi-consciousness

Prophet: he looks at your name on your badge Alcatraz, right, that's your name? Alcatraz...Destiny's a bitch, huh? It'll be on you now, son.