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 in the level The Covenant and after you have started being attacked by the Flood, you can melee or teabag the Prophet of Truth a couple of times, causing him to fall through the platform on which his body rests. When he falls beneath the platform you can see him, but you cannot reach him.