I looked up how to get this skull on Halo wiki, started up a game on The Covenant on Normal, followed the instructions to make the skull spawn to the letter, Got the skull, and ran with it to the end of the level and waited for the little cut scene with Cortana's holo-gram.. and didn't get anything.
Waited for the next level to start, save/quit, went to check my achievements, and saw it did not record anything. :(
Anyone have any ideas on why this might not be working? O.o