13 Skulls are confirmed thus far. These skulls and their effects include:
- Iron - Cannot revive/be revived
- Black Eye - Shields do not recharge until you melee enemies
- Tough Luck - Enemies berserk, always dive, and never flee
- Catch - Enemies are grenade happy
- Fog - Motion sensor is disabled
- Famine - Weapons drop less ammo
- Thunderstorm - Enemy abilities upgraded
- IWHBYD - Rare combat dialog becomes more common
- Grunt Birthday Party - Head shot a grunt for a surprise
- Cowbell - Acceleration from explosions increased
- Blind - HUD and first-person arms/weapon hidden
- Mythic - Increased enemy health
- Tilt - Enemy resistances/weaknesses enhanced
