Skulls are hidden objects in each campaign level (or now in the new multiplayer levels , I think) When you pick up a skull you "unlock" it, in the sense that you can now use it's ability in campaign. When you activate the skulls they actually have a negetive effect on you, and make the game even harder. The reason why people would do this is because it adds a bonus when you are playing to get a score ( You get an achievement when you get a certain score).
Also, if you find all thirteen skulls, you can unlock the Hayabusa armor.
Or maybe you mean, "Which are the skulls in Halo 3?". Well, here's a list.
Iron
Black Eye
Tough Luck
Catch
Fog
Famine
Thunderstorm
Tilt
Mythic
Blind
Grunt Birthday Party
Cowbell
IWHBYD (I Would Have Been Your Daddy)
you can get all of the skulls on multiplayer. =)
if you want to know where the skulls are all you have to do is go to youtube and put in "halo 3 odst mythic skulls" and it will show you how to get to them
ok, go to youtube and type in "how to find all halo 3 skulls" then watch the video and get the skulls. yes, its that simple.
By getting skulls.
get all skulls
Find them your self
Go to Halopedian.com. They have the largest source of everything Halo.
If i am correct then it is the same as the skulls being off.
THey get you an achievement, and they get you the Hayabusa helmet.
archon armor
All skulls are automatically unlocked for Halo 3: ODST due to gameplay reasons.
Sorry there arn't any, The only skulls are in "Halo 3" Which are used in "Halo 3 O.D.S.T."