Mephos is an Avian who desires to conquer the surface world.
When the Avians freed themselves from Y'Lyntian rule, they lived peacefully in the sky. But many believed that they should rule over the surface world, not hide from it. Mephos was the main instigator of the ideals and led a rebellion to control the city. He ultimately lost and was banished to the surface world, stripped of his wings. His former friend, Raptarr, kept an eye on him to make sure he didn't do any harm and prevent him from returning to the city.
Mephos eventually managed to build himself metallic wings that enable him to cause pain to his enemies and form a diamond drill. He fights against Raptarr and steals his crystal diadem, which would enable him to locate the city. There, he met up with his followers who had managed to secure the Council chamber. He intended to use the remnant crystal sun fragments as a weapon to destroy anything on the surface. He test tried on a mountain and was about to target Seattle when Raptarr and the Turtles, equipped with flight tech packs, stopped him. They tricked him into catching the control crystal too close to the main core, electrocuting him. Once again, he was defeated, stripped of his wings again and then imprisoned in jail.