What does a hidden skull do on Halo 3?

There are a total of 13 hidden skulls to be found in the Campaign mode of Halo 3. I will list the names and description of each one:

Iron- If you die while playing alone, you will restart at the beginning of the level (you cannot revert to the last checkpoint at any time). If any player dies while playing co-operative, you will reset to the last checkpoint

Black Eye- Your Energy Shield will not recharge until you perform a melee hit. The Energy Sword and Gravity Hammer's R-Trigger attacks count as a melee attack. Hijacking also counts as a melee attack.

Tough Luck- Enemies dodge grenades, needles, and vehicle assaults with a far greater success rate. However, plasma and spike grenade sticks are still possible at close range, and from behind.

Catch- All AI-controlled units, friend or foe, throw grenades more often, where applicable

Fog- Your motion sensor is no longer active.

Famine- All weapons that are dropped by enemies or are scripted to spawn on the ground (e.g., near a downed Pelican or group of dead bodies) have halved or reduced ammunition or charge. However, weapons found in cases still have the regular amount of ammo.

Thunderstorm- All enemies are promoted to the highest available rank. The number of shielded Flood elite combat forms is also increased.

Tilt- All enemy weaknesses and resistances double. Example: Brute armor is weaker to plasma weapons and stronger against projectile weapons.

Mythic- All of the Covenant you encounter have twice the amount of health.

Blind- You lose all your HUD when this skull is turned on. You won't see your weapons, shield, radar, etc.

Grunt Birthday Party- Arguably the most popular Skull, if a Grunt is shot in the head with any headshot-capable weapon (such as the Battle Rifle, Beam Rifle, Carbine, and Sniper Rifle) or a Magnum, confetti explodes out of it (the explosion causes minor damage to nearby enemies), and a sound clip of children cheering from the game Viva PiƱata can be heard in the background.

Cowbell- The force of explosions is increased immensely.

IWHBYD (I Would Have Been Your Daddy)- Unlocks new, often humorous, dialogs for the AI. Such as "THIS IS SPARTA!" or "Who wants ice cream!?" for some marines or "Pie is good" said by some Brutes. Sometimes a Brute may even say "He was my lover!" when you kill a nearby Covenant soldier, and Grunts may say "Stay close to me, Nipple-mate!" to another Grunt when they are suspicious that enemies may be nearby, or "Thank you for choosing Air Blue-Ball" as they go Kamikaze, Elites sometimes make humorous remarks when you exchange weapons or stare at them for long periods of time. Remarks like "I shall name my next child after you" or "I, I, I think you're sexy, too." Another comment from a marine is "So you're gonna take my girlfriend, too?" if you exchange weapons with him.

by the way theres been a hidden skull found in the mission cortana but no one nos what it does so far