Now, how to find the skulls. Each level in Halo Wars has an optional objective where you are asked to kill a certain number amount of a certain unit type, such as killing 100 Grunts in the first mission, Alpha Base. Once this objective has been completed, the skull location will flash on the mini-maps for a couple seconds, so it's important to watch the map as you complete the objective. To simplify this, the objective will have a counter that pops up every time you kill a unit of the designated unit type. Example, the HUD will show "56/100 Grunts Killed" or something similar. Here is the information I have managed to find thus far. One last thing to note, is that the skull will not appear in it's location until the objective has been met.


Mission 1: Alpha Base
Skullname: Look Daddy!
Location: On embankment near the downed ghost where last set of marines are found (Indie)
Kill 100 grunts.
special effect when grunts die their methane tank always shoots off

Mission 2: Relic Approach
Skullname: Grunt Birthday Party
Location: Southwest of your starting base location in front of damaged gate (Indie)
Kill 20 Jackals
special effect grunts explode into confetti

Mission 3: Relic Interior
Skullname: Cowbell
Location: In front of the main console where Forge and Anders are initially pinned down (Indie)
Kill 45 Hunters
special effect destruction physics magnified

Mission 4: Arcadia City
Skullname: Wuv Woo
Location: Very eastern part of the map near statue of man holding book (Indie)
Kill 50 Elites
special effect scarabs shoot rainbow made of pure love

Mission 5: Arcadia Outskirts
Skullname: Fog
Location: north west of map on top the highway near two buildings (raptor00)
Kill 5 Wraiths
special effect no minimap final score will increase

Mission 6: Dome of Light
Skullname: Sickness
Location: In the hanger behind your base (raptor00)
Kill 50 Banshees
special effect all player units have 50% fewer hitpoints final score will increase

Mission 7: Scarab
Skullname: Rebel Sympathizer
Location: Next to the power node to the west of the scarab (raptor00)
Kill 10 Locusts
special effect non-player units get 25% more hitpoints final score will increase

Mission 8: Anders' Signal
Skullname: Rebel Supporter
Location: On cliff edge beside a crevas in the northern part of the map (raptor00)
Kill 750 Infection Forms
special effect non player units do 50% more damge final score will increase

Mission 9: The Flood
Skullname: Rebel Leader
Location: By the downed pelican to the west of your starting base (raptor00)
Kill 20 Flood Stalks
special effect non-player units star with extra veterancy final score will increase

Mission 10: Shield World
Skullname: Catch
Location: On the island in the south east section of the map (you need air units to reach it) (raptor00)
Kill 350 Swarms
special effect all player units recharge unit abilities in half the time final score will be decreased

Mission 11: Cleansing
Skullname: Sugar Cookies
Location: North east section of the ship in between both platforms (you need air units to reach it) (raptor00)
Kill 100 Sentinels
special effect all player units have 50% more hitpoints finla sore will decrease

Mission 12: Repairs
Skullname: Boomstick
Location: North east section of the ship above the central stucture (raptor00)
Kill 12 Spirit Transports
special effect 5%chance any non-player unit blows up upon death with explosive area of effect damage final score will decrease

Mission 13: Beachhead
Skullname: Pain Train
Location: In the first area you start in near the flood buildings (west of map) (raptor00)
Kill 10 Bomber Forms
special effect all player units train 50% faster final score will decrease

Mission 14: Reactor
Skullname: Bountiful Harvest
Location: North-most part of the map near the second covenant base (raptor00)
Kill 20 Vampires
special effect supplies come in 25% faster final score will decrease

Mission 15: Escape
Skullname: Emperor
Location: North East of the map near a hot water spring and a flood base (raptor00)
Kill 3 Scarabs
special effect all player powers recharge in half the time final score will decrease

Credit goes to: The Interwebs!