Halo Wars skulls
From Halopedia, the Halo wiki
How to find them
Each level in Halo Wars has an optional objective where you are asked to kill a certain number 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-map for a few 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. For example, the HUD will show "56/100 Grunts Killed"; when you have killed 100 Grunts for that objective, the skull will appear.
Level: Alpha Base
Special Objective: Kill 100 Grunts.
Location: On the embankment near the downed Banshee where last set of Marines are found.
Icon: A skull with a dummy.
Note: This is perhaps the hardest skull to obtain, since most times, a player kills only 95-98 Grunts before the end of the mission. Try using only Warthogs to splatter the Grunts before the A.I.s kill them. Also, while inside Alpha Base, do your best not to destroy the Spirit transports, as they drop additional Grunts.
Grunt Birthday Party
Level: Relic Approach
Special Objective: Kill 20 Jackals.
Location: At the south end of Alpha Base.
Icon: A skull marked with Grunt mask and pink party hat.
Effect: Grunts explode into confetti after being killed.
Level: Relic Interior
Special Objective: Kill 45 Hunters.
Icon: A skull covered with a cow pattern (white skull with black splotches).
Effect: Destruction physics magnified.
Level: Arcadia City
Special Objective: Kill 50 Elites.
Location: Very eastern part of the map near statue of man holding book.
Icon: A purple skull marked with a pink lipstick kiss on its head.
Effect: Scarabs shoot a rainbow made of pure love and hearts.
Level: Arcadia Outskirts
Special Objective: Destroy 5 Wraiths.
Location: North west of the map, on top the highway near two buildings.
Icon: A light turquoise mist covering the skull.
Effect: No minimap, final score will increase.
Level: Dome of Light
Special Objective: Destroy 50 Banshees.
Location: In the hangar just behind and to the right of the UNSC Base.
Icon: A green mist covering the skull.
Effect: All player units have 50% fewer hit points, final score will increase.
Special Objective: Destroy 10 Shua'ee-pattern Locusts.
Location: Next to the power node to the west of the Scarab.
Icon: A skull marked with a dark green fist on its forehead.
Effect: Non-player units get 25% more hit points, final score will increase.
Level: Anders' Signal
Location: On cliff edge beside a crevice in the northern part of the map.
Icon: Same as Rebel Sympathizer, only the fist emblem is red.
Effect: Non-player units do 50% more damage, final score will increase.
Level: The Flood
Special Objective: Kill 20 Flood Stalks.
Location: By the downed Pelican to the east of your starting base. You will need air units, as ground units can't get to the cliff the skull is on.
Icon: Same as Rebel Supporter, only the forehead is flaming.
Effect: Non-player units start with extra veterancy, final score will increase.
Level: Shield World
Special Objective: Kill 350 Swarms.
Location: On the island in the south east section of the map (you need air units to reach it).
Icon: A skull marked with two baseballs on each side.
Effect: All player units' abilities recharge in half the time, final score will decrease.
Special Objective: Destroy 100 Sentinels.
Location: North east section of the ship in between both platforms, or, between the tail-fins of the ship.
Icon: Normal skull, only it looks like a skull-shaped cookie.
Effect: All player units have 50% more hitpoints, final score will decrease.
Special Objective: Destroy 12 Spirit Dropships.
Location: North east section of the ship above the central structure.
Icon: A skull with its cranium exploding.
Effect: There is a 5% chance that any non-player unit blows up upon death with explosive area of effect damage, final score will decrease.
Location: In the starting area, near the Flood buildings (west of the starting area).
Icon: A skull that is completely crimson.
Effect: All player units train 50% faster, final score will decrease.
Special Objective: Destroy 20 Bkowe'nei-pattern Vampires.
Location: North-most part of the map near the second Covenant base.
Icon: Normal skull, only different colored fruits are on each side of skull.
Effect: Supplies come in 25% faster, final score will decrease.
Special Objective: Destroy 3 Scarabs.
Location: North east of the map near a hot water spring and a Flood base.
Icon: A skull marked with a crown.
Effect: All player powers recharge in half the time, final score will decrease.
The Wuv Woo skull is a reference/continuation of a recurring joke in two of Ensemble Studios' previous games. In Age of Mythology, entering the cheat code "wuv woo" spawns the Flying Purple Hippo unit, while in Age of Empires III the code "wuv woo vol.2" spawns the Flying Purple Tapir. Like Wuv Woo skull, both units have an exceedingly cutesy appearance.