Invincibility

Invincibility is a new piece of Equipment featured only in Halo 3. It gives the user a brief period of Invulnerability lasting for about 10 seconds. In order to obtain it, the player is required to kill a Brute Chieftain without having it activated. Like the Gravity Hammer is to the Fist of Rukt, this Equipment originates from Tartarus's Modified Invincibility.

Operation
When activated the player is covered in shining white bands. It is known to be possessed by all Brute Chieftains that wield a Gravity Hammer unhindered, (not the Fuel Rod Gun Brute War Chieftain, whether or not they wield a Plasma Cannon) shooting them is simply useless, as you cannot harm them in any way, while it is activated. It is true, though, that assasinations are still effective.

In Campaign, it can be found on some dead Brute Chieftains that haven't used the power-up, but this is a rare occurrence as they will usually activate it the instant they are attacked. Curiously, if you melee a Brute in the back (assassination hit) while the invincibility is active, it will still kill him as if it wasn't active. In addition to being obtainable from dead Brute Chieftains, there is an Invincibility in the level "Halo" on Easy, Normal, and Heroic difficulties. After going up the staircase, go to the left and into the tunnel. This makes the battle with 343 Guilty Spark a lot easier, since you cannot get hit by the powerful beam. Note: This is only found on difficulties Easy, Normal and Heroic. It cannot be found there on Legendary to make the level more of a challenge.

It has a similar appearance to the Cloaking equipment, except that it glows a gold color, rather than a cool blue color. It's symbol on the HUD is two overlapping, intersecting ovals with a dot at the center.

It is not present in Multiplayer, unless you edit game-play options where you can make players invincible. Note: The Guardians can still kill you, apart from them your only real danger is from being assassinated from behind.

In campaign, though you will be invulnerable from almost all forms of attack, you may still die from falling or having a large object such as a Scorpion crush you, or by having tons of Infection Forms infect you. Invulnerable characters like the Arbiter who become enemies by you after you kill them can also kill you very easily.

By Modding, you can place the equipment on Isolation or Cold Storage. Apparently, this equipment was intended to be available only on Flood multiplayer levels.

Disadvantages

 * Players or AI who have activated the Invincible equipment can still be killed by being assassinated, crushed by large objects, or falling from a high altitude.
 * It only lasts for a 10 seconds, so it should only be used as a last resort, not the first chance you get.
 * When the equipment is deployed, it makes the player glow a gold/white/purple color, much like an Overshield, therefore making you more visible when used and might attract unwanted attention towards you.
 * It is still visible even with the aid of cloaking or (probably) Active Camouflage.

Trivia

 * The easiest way to get one, is to kill a Brute Chieftain in one shot or from far away using a Sniper Rifle, which obviously doesn't give him the chance to use it. Notable ways include the Spartan Laser and Rocket Launcher, and on easier difficulties, a Beam Rifle or Sniper Rifle head shot. Grenades hardly ever make a one hit on them, with the exception of the Incendiary grenade.
 * The Invincibility has an analogous effect to Tartarus's shields, which he uses in the last level of Halo 2.
 * It is most useful when used for running at large groups of enemies, as they are unable to harm you without making assassination.
 * The Invincibility symbol resembles a Helium atom. This may be because the power that is exerted by this equipment can be compared to that of an atom,(or sub-atomic and elementary particle) when disintegrated, 'split' or charged.
 * It is not present in Multiplayer, even Forge, due to it being overpowered. Though it can be obtained through modding.
 * If you make the Arbiter an enemy by killing him/allies, and use the Invincibility, he can still kill you with one plasma bolt.
 * Following in the ways of Tartarus, only Brute Chieftains with Gravity Hammers utilize Invincibility in Halo 3 Campaign.
 * This is very similar to the the shield players get in Rocket Race as they can only be meleed or assasinated from behind.