Orbital Skull (achievement)
From Halopedia, the Halo wiki
Orbital Skull is an achievement in Halo 3 and Halo: The Master Chief Collection. It is unlocked by finding the hidden skull on the multiplayer map Orbital.
The achievement is worth 25 Gamerscore in the original game and 5 Gamerscore in The Master Chief Collection.
To find this map's skull, one has to go to Forge mode, and fly high above the shaft of the map, and a very high column near the Sniper Rifle, Power Drain and dual Okarda'phaa-pattern plasma rifles hallway should have the skull. To reach the skull, however, one has to find which part of the column a player can stand on. Once they succeeds, the player must crouch walk to the skull, in order not to fall off easily.
Another method to grab the skull is to fly up to where the skull is, and then make sure you are facing the skull and you are near it, now go back to player mode and immediately press and hold the right bumper, (if on default game controls), if done correctly, you should have the skull in your hands. Another way to do it is to spawn a weapon holder, and use it to drag the skull out of the area.