There is a skull located in every campaign mission in Halo Wars for the Xbox 360. Here is a list how to get the skulls, and what their effects are.
Mission 1 Alpha Base – Kill 100 Grunts – Located on the left side of the final battle area before the Alpha Base. Effect: Look Daddy! – When Grunts die their methane tank always shoots off
Mission 2 Relic Approach – Kill 20 Jackals – Located inside Alpha Base on the south side. Effect: Grunt Birthday Party – Grunts explode into confetti when they die
Mission 3 Relic Interior – Kill 45 Hunters. Located at the back on a circular platform where you have to save Forge & Anders. Effect: Cowbell – Destruction physics are increased
Mission 4 Arcadia City – Kill 50 Elites. Located on the right side of the map right below the Covenant base past the top of the stairs. Effect: Wuv Woo – Scarabs shoot rainbows
Mission 5 Arcadia Outskirts – Kill 5 Wraiths. Located at the top left corner of the map. Effect: Fog – The mini-map is disabled, and your final score will be increased
Mission 6 Dome Of Light – Kill 50 Banshees. Located inside of the large hangar behind your base. Effect: Sickness – All player units have 50% fewer hit points, and your final score will be increased
Mission 7 Scarab – Kill 10 Locusts. Located on the left side of the map at the edge of the battlefield area on a ledge by the canyon, near a power node. Effect: Rebel Sympathizer – Non-player units get 25% more hit points, and your final score will be increased
Mission 8 Anders’ Signal – Kill 750 Infection Forms. Located on the far right of the map on the ridge. Effect: Rebel Supporter – Non-player units do 50% more damage, and your final score will be increased
Mission 9 The Flood – Kill 20 Flood Stalks. Located near the base on the plateau close to the burning Covenant ship. Effect: Rebel Leader – Non-player units start with extra veterancy, and your final score will be increased
Mission 10 Shield World – Kill 350 Swarms. Located on an island in the southeast, requires a flying unit to reach it. Effect: Catch – All player units recharge unit abilities at half normal speed, and your final score will be decreased
Mission 11 Cleansing – Kill 100 Sentinels. Found floating in front of the ship, requires a flying unit to reach it. Effect: Sugar Cookies – All player units have 50% more hit points, and your final sore will be decreased
Mission 12 Repairs – Kill 12 Spirit Transports. Located on the large structure on the Spirit of Fire, a flying unit is required to reach it. Effect: Boomstick – 5% chance that non-player units blow up upon death for explosive aoe damage, and your final score will decreased
Mission 13 Beachhead – Kill 10 Bomber Forms. Located on a Flood vent near the area where the Flood come from as you try to enter the Pelican. Effect: Pain Train – All player units train 50% faster, and your final score will be decreased
Mission 14 Reactor – Kill 20 Vampires. Located to the back left of the second Covenant base. Effect: Bountiful Harvest – Supplies arrive 25% faster, and your final score will be decreased
Mission 15 Escape – Kill 3 Scarabs. Located behind the Flood base, at the very back of the map. Effect: Emperor – All player powers recharge in half the time, and your final score will be decreased
