Life After Death
From Halopedia, the Halo wiki
On the level Halo in Halo 3, shoot Sergeant Johnson in the head to make him fall for 5 seconds. Then run into the Control Room while Johnson is down, after the movie when Johnson dies. If done correctly, he will still be alive. Although after 3 seconds a Flood infection form will jump onto him. On higher difficulties instead of him getting infected, the form on him will disappear. Then he will get up and help you eliminate all the flood there, although he is supposed to be dead. He disappears as soon as you leave the control room's entrance door.
