How do the skulls work in Halo 3?

[cornio89]

I don't get it..

 

[White Shinobi]

The skulls work to get different costume options on story/campaign mode.

 

[Alex M]

Each skull has its own little "bonus" so say if you got the skull Catch and you turn it on the enemies will throw more grenades. Each skull has its own thing that will happen, one of them gets rid of your HUD and gun.

But first you need to find them.