Skulls can be earned by completing different task in the specific mission; you can have them as optional objective. Once you compete the objective you will earn the skull. Earning different skulls will give you some extra bonus effects which can be very useful throughout your mission progress.
Skulls Detail:
Below we have listed the entire available skull with the mission detail so you can find and complete these objectives and earn the skull.
Grunt Birthday Party
- Mission: A New Enemy
- What to Do: Capture Four of Decimus’ Power Nodes. Completion Earns a Skull!
- Effect: Killed Grunts explode into a shower of confetti, bringing joy to the battlefield—if you’re not a Grunt, that is.
Paintball
- Mission: Ascension
- What to Do: Destroy the Hidden Facility’s Four Structures. Completion Earns a Skull
- Effect: Marine grenades and Grunt mines splatter the environment with color (as well as deadly shrapnel). Time to paint the town!
Pain Train
- Mission: Ascension
- What to Do: Destroy the Banished Base. Completion Earns a Skull (on island)
- Effect: All player units train 50 percent faster. Final score decreases.
Bountiful Harvest
- Mission: One Three Zero
- What to Do: Don’t Let Any of the Four Prison Squads Die.
- Effect: Supply from Supply Pads arrives 25 percent faster. Final score decreases.
Emperor
- Mission: The Cartographer
- What to Do: Destroy Both Minibases in the Central Chamber.
- Effect: All player Powers recharge in half the time. Final score decreases.
Pestilence
- Mission: Lights Out
- What to Do: Rescue All 10 Squads of UNSC Prisoners.
- Effect: All player units slowly lose health from the moment they’re built. Final score increases.
Total Annihilation
- Mission: From the Deep
- What to Do: Mission completed without Spartan downed
- Effect: Spartan special-ability Slams create an even more powerful explosion upon landing. Final score decreases.
Sickness
- Mission: Hold the Line
- What to Do: No barricades destroyed
- Effect: All player units have 50 percent fewer hit points. Final score increases.
Firestarter
- Mission: Hold the Line
- What to Do: Particle Cannon stays above 50 percent health
- Effect: Grunts and Marines explode upon death, inflicting damage on enemy units. Final score decreases.
Things That Make You Go Boom
- Mission: Under the Dark
- What to Do: All three Brute Warlords eliminated
- Effect: Hero units unleash a nuclear explosion upon death. Final score decreases.
Wuv Woo
- Mission: The Foundry
- What to Do: Scarab survives the mission
- Effect: Scarab and Locust beam weapons shoot rainbows made of pure love and hearts. Maybe Atriox isn’t such a bad buy after all!
Sugar Cookies
- Mission: The Foundry
- What to Do: Scarab health doesn’t drop below 25 percent
- Effect: All player units have 50 percent more hit points. Final score decreases.
After Party
- Mission: The Halo
- What to Do: Bravo Base rescued
- Effect: Anytime a Firebase is destroyed, the player who owned it gets a tank and infantry unit. Final score decreases.
Nightmare
- Mission: Last Stand
- What to Do: Control Room Shield takes less than 25 percent damage
- Effect: No checkpoints or saves on any difficulty. Campaign only. Final score increases.
Shadow
- Mission: Last Stand
- What to Do: Both Brute Warlords and their reinforcements eliminated
- Effect: All infantry units start cloaked. Final score decreases.
Bountiful Harvest’s description is incorrect, with “ANY prison squads”; it should be “4”, because you can lose up to 3 without failing the objective – the 4th’ll do you in.