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.

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 Brutes, or the Brutes with a Plasma cannon) giving them a significant tactical advantage, as you cannot harm them in any way.

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".

At the top of the Control Room tower, where you have to fight hordes of Flood, there is an Invincibility. After going up the staircase, go to the left and into the tunnel. It will be there. 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, as well as other players who assassinate you.

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 betrayed by you after you kill them can also kill you easily.

Trivia

 * A player is able to take a Brute Chieftain's Invincibility if they assassinate, head shot, or in some way kill the Chieftain before he has a chance to activate it and use it.
 * The easiest way to get one is to kill a Brute Chieftain in one shot, 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 Fire Bomb.
 * Despite being invincible, a player can assassinate a Brute Chieftain, or another player successfully.
 * The Invincibility has an analogous effect to Tartarus's shields.
 * It is most useful when used for running at large groups of enemies, as they are unable to harm you without making assassination.
 * It has since been argued by players as to why it was constructed in Halo 3's Campaign, as it is not a legitimate piece of equipment by no means.
 * 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.
 * If you make the Arbiter an enemy by killing him/allies enough, and turn on Invincibility, he can still kill you with one plasma bolt.

Disadvantages

 * Players or AI who have activated the Invincible power-up can still be killed by being assassinated, crushed by large objects, or falling from a large height.
 * It only lasts for a short period of time(around 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 active camouflage or cloaking.