How can you get iwhbyd in Halo 2?

To unlock the "I Would Have Been Your Daddy" (IWHBYD) skull in Halo 2, you need to complete the mission "Quarantine Zone" on any difficulty. During the level, after you encounter the first Flood, look for a hidden area on the left side of the map. You’ll need to jump onto a structure and then make your way to a platform where the skull is located. Once you find it, simply walk over to collect it.