Unknown Skull?
Unknown Skull Effect: Currently, it seems as if this skull does not unlock anything, and furthermore does not have any effect in the game. It’s true name (if it has one) is not known.
Unknown Skull Location:
The Unknown Skull can be found on the level, “Cortana”, in the room with the 3 generators you must destroy. The skull is on the ceiling of this room (just above the door where you walk in for the first time).
This is not the IWHBYD Skull in Halo 3, and does not unlock any new Gold or Silver Skulls. Its purpose remains unknown.
UPDATE ON UNKNOWN SKULL:
Turns out that this is actually just another “Blind Skull“. Apparently, Bungie threw in two blind skulls into Halo 3 for some reason. This was discovered with the help of an anonymous reader who managed to get a look into the game code (Note: The “hc” map is “Cortana”).
