Floating plasma grenade
From Halopedia, the Halo wiki
In Assault on the Control Room (Level), when you get to the first lift, stand behind the sign that says Press X to Activate. Lift and throw a Plasma Grenade into the center of the lift. The lift will go down, but the grenade will float in front of you. The grenade will harm you, so if you have low health, you will die after it explodes.
