A full list of all the skulls in Halo 2 has been provided to Halo2Source by abhinav; one apparently unlocks a level of difficulty higher than Legendary. The list has this as confirmed but I haven't seen confirmation as yet.

UPDATE: There's also voluminous info on the search for skulls in the High Impact Halo forum. Currently their Skulls page links to the latest skulls post by Halo-2-Xlink. According to HIH, it appears that rather than unlocking a separately accessible difficulty level, the Mythic skull makes Flood invicible while in vehicles, but also makes the whole game on Legendary more difficult. Thanks Gil27 and Hawk7886.