HI BEN

On the Halo 2 campaign level "Regret", there is an easter egg with the words "HI BEN" written on the ceiling of a hidden room that is dedicated to Benjamin Burnley of Breaking Benjamin, who wrote the song Blow Me Away that appears in Halo 2. There is also, in purple, the "rock on" hand sign on the right hand corner.

Walkthrough
You will need:
 * A Sniper Rifle or Beam Rifle
 * The Sputnik skull

Start the level Regret and proceed until you kill the Drones and Elites. Go outside and to your right you will see a temple with a broken bridge leading to it. Explosive Jump there and go to the room to your right. Activate your Flashlight and go to a left corner and zoom in with your scope and you will see the message.'

Or 

Continue the game until you reach the platform where the Hunters are dropped and walk to the side of the platform that is facing the temple with the broken bridge. Throw a grenade at your feet and jump and you should be launched across and in through the Balcony window on the other side. Then, simply follow instructions from the first method above.

Note: You do not require the Sputnik skull for this method.