The Custom Powerup is an object in Halo 3, similar to the Active Camouflage and the Overshield.
Overview
This object is only found in Forge. What it does is decided by a player in Forge. It can affect many player attributes, such as damage resistance, gravity, damage, etc., including multiple simultaneous effects. It can make a player an "instant Juggernaut", or even negatively affect a player. It will appear as a floating yellow orb. [1]