Skulls are a fan favorite addtion to the Halo franchise and they have been in every game in one shape or another since Halo 2. The skulls activate specific adjustments to gameplay and each skull alters the game experience in its own unique way.
Some will make it more challenging, others will cause enemies to behave differently and some alter game play physics. Its up to you to try each one out and decide which ones you like to play with! Unfortunately, unlike some previous games, these are not hidden throughout the campaign in halo 4 but are instead unlocked from the start. They can be activated from the campign menu.
Your shields do not recharge unless you melee enemies.
Your entire HUD, weapon and arms completely removed.
Enemies will throw and carry more grenades than usual.
Your motion sensor is disabled.
Acceleration of objects from explosions is increased.
Weapons picked up from the ground have less ammo.
Grunt Birthday Party
Headshotting a grunt will cause it to explode.
In co-op mode a death of one player has both return to the previous checkpoint. When playing solo a death will mean restarting the level.
Enemies and allies get additional humorous dialogue.
Enemies have increased health.
All enemies will be the next rank up from usual.
Enemy resistances and weaknesses are increased.
Enemies always go berserk, dive out of the way and never flee combat.