Invincibility
From Halopedia, the Halo wiki
Invincibility is a powerup to be featured in Halo 3.
It confers the user with a brief period of invulnerability, functionally and aesthetically similar to Tartarus's energy shield. It is known to be possessed by some or all Brute Chieftains in Halo 3, giving chieftains a brief period of time where they may chase after the player with the Gravity Hammer unhindered, giving them a significant tactical advantage. It is also present in multiplayer, although its appearances will be infrequent, since the powerup will be too "cheap" for some players.
