Heretic leader glitch
From Halopedia, the Halo wiki
The Heretic leader glitch is a Halo 2 glitch that allows players to kill the Heretic Leader on The Arbiter, one level before you are supposed to. The glitch should not be attempted by players who are trying to beat the game, as it will render the rest of the level unplayable.
Play the level The Arbiter normally, keeping a charged Energy Sword with you at all times. Eventually, you will come to a room where you can see the Heretic Leader out on a Banshee Launchpad through a sheet of ornate glass. The reticule will turn red if you get close and move it over him. Get out your Energy Sword, and start attempting to lunge at the Heretic Leader through the glass. Crouch, jump, and crouch-jump, tapping the right trigger the whole time. It may take a few tries, but eventually, the Arbiter will lunge at the Heretic Leader, going straight through the glass.
Quickly grab his Banshee before he can pilot it, pick off his men, and the Heretic Leader will stop moving. He is completely invulnerable to all forms of damage, though you can kill him by knocking him off the ledge onto the pipeline below it. He screams the same way as when he is killed on the next level, The Oracle.
He will also die if you hijack his Heretic Banshee, but if you go anywhere down towards where he fell, you will die.
This glitch will also affect the Phantom that comes to support you. The Phantom will simply sit idly, firing at the Heretic Leader (or his dead body); it will not trigger any enemies to appear, and it will not trigger the door at the end of the level to open. In other words, you will not be able to proceed past this point after you do the glitch.