Four Banshees on The Package
From Halopedia, the Halo wiki
This Easter egg is especially useful for playthroughs on Legendary difficulty due to the ability to skip past a large portion of the level. However, care must be taken not to activate the Easter egg before destroying the two anti-air guns early in the mission, as activating the Easter egg will make the two guns invulnerable and prevent progress through the mission. If completing the mission is a goal, destroy the anti-air guns first then return to the mission's start and activate the Easter egg as normal.
First off, start The Package with at least two players (skulls are irrelevant). Clear the first area, but make sure you have at least one Type-32 Ghost left driveable. Use the Ghost to push the crate (pictured below) away, showing nothing but wall. Keep at least 1 player near this wall while the other(s) claim the Scorpion. Clear the tower and some forces nearby which may kill you.
Jump in the water behind the rock (pictured below), a soft-kill barrier is active here, and turn the green switch present. When this switch is turned the wall cleared earlier now holds a similar switch, let player 2 turn it quickly, because it will disappear forever after around ten seconds.
When this switch is also turned at first glance, nothing happens (you won’t hear any sound from it). But on the other building’s roof now stand 4 Banshees to be flown throughout the rest of the level. When these Banshees are all destroyed, they can only be returned by restarting the level and following the steps all over again.