On each mission complete the task and then search the indicated location to find the corresponding skull. When you have the skull you will be able to activate it's special effect at the 'Pause' men. Collecting the skulls is also a requirement for the 'Alas, Poor Andrew Thomas' and 'Graverobber' achievements.
Mission 1 Skull - Alpha Base:
Kill 100 Grunts. The skull appears east of the entrance to Alpha Base.
Mission 2 Skull - Relic Approach (Look Daddy!):
Kill 20 Jackals. The skull appears at the bottom of your base.
Mission 3 Skull - Relic Interior (Grunt Birthday Party):
Kill 45 Hunters. The skull appears where you rescue Anders and the Marines.
Mission 4 Skull - Arcadia City (Wuv Woo):
Kill 50 Elites. The skull appears south of the Covenant base on the east side of the map.
Mission 5 Skull - Arcadia Outskirts (Fog):
Kill 5 Wraiths. The skull appears at the northern-most part of the highway on the west side of the map.
Mission 6 Skull - Dome of Light (Sickness):
Kill 50 Banshees. The skull appears on the small platform where you lock down the third Rhino.
Mission 7 Skull - Scarab (Rebel Sympathizer):
Kill 10 Locusts. The skull appears by the northeastern most power node.
Mission 8 Skull - Anders' Signal (Rebel Supporter):
Kill 750 Infection Forms. The skull appears north of where you set up base.
Mission 9 Skull - The Flood (Rebel Leader):
Kill 20 Flood Stalks. The skull appears just east of your base, near a downed Pelican.
Mission 10 Skull - Shield World (Catch):
Kill 350 Swarms. The skull appears on a small island between the southern Pylons (need an air unit).
Mission 11 Skull - Cleansing (Sugar Cookies):
Kill 100 Sentinels. The skull appears at the back end of the ship where the cleansing shields start (air unit is required).
Mission 12 Skull - Repairs (Boomstick):
Kill 12 Spirit Transports. The skull appears near the back of the ship, just above the two interlocks (air unit is required).
Mission 13 Skull - Beachhead (Beachhead):
Kill 10 Bomber Forms. The skull appears in the small valley west of where you start the level.
Mission 14 Skull - Reactor (Bountiful Harvest):
Kill 20 Vampires. The skull appears at the bottom of the third ramp between the second Covenant base and the wall of the mountain.
Mission 15 Skull - Escape (Emperor):
Kill 3 Scarabs. The skull appears in the middle of the north part of the map in the middle of the Flood.
Added by: Sanzano May 15th 2009, ID#4426