What are skulls in halo?

Skulls are hidden in every level and can be found and picked up.

Once picked up they are unlocked and can be activated.

Once activated they make your game harder and more challenging.

There are 6 skulls located in the multiplayer, on these maps:

Heretic

Longshore

Citadel

Assembly

Orbital

Sandbox

Gold Skulls, found in campaign:

Iron

Black Eye

Tough Luck

Catch

Fog

Thunderstorm

Tilt

Mythic

Silver Skulls

Blind

Grunt Birthday

Cowbell

I Would Have Been Your Daddy