Effect: Currently it seems that this skull does not unlock anything and furthermore does not have any affect on the game. It’s real name (if it has one) is unknown.
Location: The Unknown skull can be found on the Cortana level. It is in the room with the three generators that you must destroy. The skull is on the ceiling of the room (just above the door where you walk in for the first time).
Update: It turns out that this skull is actually another ‘Blind Skull’. It seems that Bungie have thrown two of these skulls into Halo 3. This was discovered by an anonymous gamer who managed to get a look into the game code.
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The unknown skull gets you a spartan laser for the begining of the final lvl if u hang on to it to the end of cortana.