On the final level of Halo 2 there are two secret Golden Heretic Banshees. To get these banshees, you have to start from the beginning of the level and go through most of the level without using any vehicles. Once you are at the part with the giant cliff, go down and the 1st banshee will be there. Fly up towards the right and the second will be there. The banshees may be slightly damaged. You can even use these banshees to fight the final boss.