The Sputnik skull is required for this.
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. Grenade jump there and go to the room to your right. Activate your Flashlight and go to the left corner. Zoom in with your scope and you will see the message.
Continue 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. Grenade jump and you should be launched across and in through the balcony window on the other side. Then follow instructions from the first method above.
- This wasn't found until after Frank O'Connor hinted at it in the July 22, 2005 Bungie Weekly Update.