Successfully complete the indicated task (optional objective) in the listed mission to unlock the corresponding skull.

Grunt Birthday Party
In the "A New Enemy" mission, capture all four of Decimus' power nodes in the second part of the map. Once you capture the third power node, it will display icons on the map with the locations of the remaining power nodes. The skull's effect is killed grunts explode into confetti.

Paintball
In The "Ascension" mission, completely destroy the Banished base in the northern area of the map. The skull's effect is marine grenades and grunt mines explode with color.

Pain Train
In the "Ascension" mission, fly to the island in the southeast corner of the map and destroy the four hidden Aug. Extrators. Note: It cannot be accessed normally; send Hornets to destroy it. The skull's effect is all player units train 50 percent faster. The final score will also decrease.

Bountiful Harvest
In the "One Three Zero" mission, do not allow any of the captured prisoners (four prison squads) to die. It is recommended to play on the Easy difficulty to make completing this challenge much less difficult. Try keeping the prisoners in the back lines to better protect them. The skull's effect is supplies from supply pads arrives 25 percent faster. The final score will also decrease.

Emperor
In "The Cartographer" mission, before going to the top conduit, destroy both Banished minibases on the west and east sides in the central chamber. The skull's effect is all player powers recharge in half the time. The final score will also decrease.

Pestilence
In the "Lights Out" mission, rescue all ten squads of UNSC prisoners. The first prisoner is found to the right at the beginning of the mission. The other prisoners can be found slightly to the right of the center of the map. The skull's effect is all player units slowly lose health from the moment they are built. The final score will also increase.

Total Annihilation
In the "From The Deep" mission, successfully complete the mission without any Spartan soldiers being killed. Simply retreat when their shields are low to prevent them from dying. The skull's effect is the Spartan "Slams" special ability creates a more powerful explosion upon landing. The final score will also decrease.

Sickness
In the "Hold The Line" mission, successfully complete the mission without losing a single barricade. It is recommended to play on the Easy difficulty to make completing this challenge less difficult. The skull's effect is all player units have 50 percent fewer hit points. The final score will also increase.

Firestarter
In the "Hold The Line" mission, have the Particle Cannon stay above 50 percent health. It is recommended to play on the Easy difficulty to make completing this challenge much easier. The skull's effect is grunts and marines explode upon death, inflicting damage on enemy units. The final score will also decrease.

Things That Make You Go Boom
In the "Under The Dark" mission, defeat all three Brute Warlords. The skull's effect is hero units release a nuclear explosion upon death. The final score will also decrease.

Wuv Woo
In "The Foundry" mission, have the Scarab survive the mission. It is recommended to play on the Easy difficulty to make completing this challenge much easier. The skull's effect is Scarab and Locust beam weapons shoot rainbows.

Sugar Cookies
In "The Foundry" mission, have the Scarab stay above 25 percent health. It is recommended to play on the Easy difficulty to make completing this challenge much easier. The skull's effect is all player units have 50 percent more hit points. The final score will also decrease.

After Party
In "The Halo" mission, defend Bravo base in the northeast corner of the map. Kill all the enemies and eventually it will become operational. The skull's effect is any time a Firebase is destroyed, the player who owned it gets a tank and infantry unit. The final score will also decrease.

Nightmare
In the "Last Stand" mission, do not allow the Control Room shield to take more than 25 percent damage. It is recommended to play on the Easy difficulty to make completing this challenge much easier. The skull's effect is no checkpoints or saves allowed on any difficulty in the campaign only. The final score will also increase.

Shadow
In the "Last Stand" mission, defeat both Brute Warlords -- one is on the southeast edge of the map, and the other is on the southwest edge. The skull's effect is all infantry units start cloaked. The final score will also decrease.