In each installment of the previous Halo games, skulls have been hidden as Easter eggs to unlock special abilities or more challenging game modes? What are all of the skulls for Halo 4, and where are they located?
There are no skulls on Halo 4 (similarly to Reach, they are unlocked from the beginning).
However, here are the names and what they do from this source. I hope this is useful.
The details for each of the skulls in Halo 4 can be seen below:
Famine - Weapons drop much less ammo.
Tilt - Enemy resistances and weaknesses are increased.
Mythic - Enemies have increased health.
Catch - Enemies are grenade happy!
Black Eye - Shields don't recharge unless you melee enemies.
Tough Luck - Enemies always go berserk, always dive out of the way, and never flee.
Iron - Co-op: Back to previous checkpoint on death.
Solo: Restart mission on death.
Thunderstorm - Major upgrade to the capabilities of enemies.
Cloud - Motion sensor is disabled.
Cowbell - Acceleration from explosions is increased.
IWHBYD - Rare combat dialogue becomes more common.
Grunt Birthday Party - Headshot a Grunt, HAPPY BOOM TIME!
Blind - The HUD and the first person arms and weapon are hidden.