Disappearing Truth
From Halopedia, the Halo wiki
In the last section of the level The Covenant (Level), after you have reached and killed the Prophet of Truth and after you have started being attacked by the Flood, you can melee attack the Prophet of Truth in the shoulder several times, causing him to disappear through the platform on which his body rests. He falls beneath the platform, and while you can see him, you cannot reach him.