The custom powerup is a customisable power-up featured in Halo 3 and Halo: Reach. that can be placed in multiplayer maps via Forge. Its appearance is similar to an active camouflage and an overshield in the way that it seems to be held together by small magnets.
The custom powerup can be customised through the multiplayer lobby's "Game Options" menu. From the settings, the power-up can be configured to affect player attributes, such as damage resistance, gravity, damage delivered, speed, etc., including multiple simultaneous effects (such as having a combination of an overshield and an active camouflage). By default, it enhances player's melee attack by a threefold and speed by 50%.
The default settings for the custom powerup (which is a speed increase) are from a never-implemented Agility Powerup.