Knock the Prophet of Regret out of his throne
From Halopedia, the Halo wiki
- Claim the Sputnik Skull.
- Play through the level Regret, on any difficulty, with the skull active.
- Grab any explosive weapons you can find -- the Needler and Fuel Rod Gun work best.
- Blast the Prophet of Regret with the Needler or Fuel Rod Cannon until his Gravity Throne flips over.
The Prophet of Regret will fall out and shoot his Plasma Pistol at you. After ten seconds, he will teleport back to his Gravity Throne.