Halo 2 IWHBYD SKULL – Achievement Guide
Complete Skull and Terminal CHECKLIST(Halo 1-4 / Google Docs)
Thanks to Cam Thiessens and Halo2Trigate this skull seems to be debunked now. Checkpoint 4 (1:51) is the trigger for the skull. Once you get this checkpoint, the skull has a 1/7 chance of spawning.
Follow the guide to the skull room and make sure you hit checkpoint 4 every time. If it doesn’t show up you will have to restart the mission.