Assassin Skull

The Unknown Skull which is called "Assassin" by some players is a hidden skull in the Halo 3 Mission Cortana.

About the skull and how to get it
The unknown skull is a hidden skull in Halo 3. In the mission Cortana, start the mission from the beginning and fight your way through the mission. When you reach the room where you have to rescue Cortana, don't rescue her yet. Instead, destroy all the flood and go to the very end of the platform. You should have a couple plasma gernades that you have bought with you throughout the mission. You will see a skinny platform and a big one right on top of it. Get on the skinny platform and then try to grenade jump using plasma gernades. Before you do this you need to get a checkpoint so you can always retry from there when you fail the first time. After succesfully grenade jumping, you will end up on the big platform. Keep following the path of the platform until you reach a corner. When you reach it, turn around and you will see the Assassin skull. If you find it, it is no longer unknown!