I just wanna clear some things up about the IWHBYD skull in Halo 2 for everyone who's had trouble with it or may attempt to get it in the future. As someone who participated in the great hunt for its triggers on HighImpactHalo many moons ago, it pains me to see so many achievement guides give out inaccurate information about it. You do not need 7 checkpoints for it. That is false. That's been known to be false for nearly 10 years. I'm not sure why it's still kicking around.
Here's the truth about triggering the skull: it's random. Each time you start the level, there is a 1 in 7 chance the skull will spawn. The only requirement (beside being on Legendary) is to spawn the jackal snipers in the alley. If the metaphorical slot machine in the game didn't hit the jackpot when the level started, then it doesn't matter how many checkpoints you've gotten, the skull won't be there.
So start the level, jump to the rooftops, peer over the edge into the alley to spawn the snipers, then run to the room that houses the skull. If it's not there, restart.
Here's a quick video I made where I spawn the skull after only getting 4 checkpoints.