Assembly Skull is a Halo 3 Achievement gained by finding the hidden skull on the map Assembly. To find the skull, one must go to Forge mode. There should be two supports structured into the top center of the map, and one of them has the skull needed for the achievement.
It is worth 25 Gamerpoints. It is unknown what, if any effect picking this skull up has aside from the achievement.
When you collect every skull in Halo 3 you will get an Vidmaster Challenge achievement, if you collect all of these vidmaster challenge you will be able to unlock "Recon" from bungie.net.