Wanna have more fun playing the campaign? Activate some skulls. They not only boost your score multiplier (gold skulls only) but they make the game more of a challenge. Here's a short description of all the skulls. If I miss one, tell me please. DX
Iron- Die and you have to start the mission all over again. On Co-op, last checkpoint for one death.
Fog- Motion Sensor be gawne.
Famine- Halves the ammo you pick up.
Mythic- All Covenant have double the health.
Tilt- Severly reduces your damage for bullets on armour and plasma on shields.
Thunderstorm- All Covenant are 1 class higher than normal. I.e. Standard Grunt turns into Armoured Grunt. Minor Elite into Major Elite, etc.
Black Eye- Melee any enemy to recharge shields
Tough Luck- Enemies always jump to evade needlers, grenades, explosives, etc. They'll never flee, either.
Catch- Enemies throw grendaes twice as much and twice as fast.
Silver Skulls:
Blind- HUD disappears
Grunt Birthday Party- Headshot a Grunt and they explode with confetti and a reassuring 'hooray!'.
Cowbell- All explosives send people flying at triple the acceleration speed.
IWHBYD- Switches chances of hearing Common dialogue and Rare Dialogue, making Rare more common.