The Mythilin are a non-corporeal race with impressive powers. During the times of Earth's Ancient Civilizations, the Mythilin visited the world, where they were worshipped as gods. Historians now believe that the Mythilin are in fact the beings that were, in mythology, heralded as gods, even possessing the same names as many mythological deities.
The Mythilin were drawn back to Earth by Nod activity in Egypt: Set arrived first, and began to wreak havoc, and it wasn't until Hellcat Squadran's arms were revealed to be unable to harm Set that Ra arrived and aided Hellcat Squadran's forces in defeating Set. When Apophis arrived, and began trying to destroy the world, other Mythilin, including Athena, Thor, Osiris, and others, arrived and were able to stop Apophis from causing damage. Then, the Mythilin left Earth, and found an alternate version of Earth, set in ancient times. They named the world Proterra, and became guardians (or, for some like Apophis and Kronos, villains) of the world.
The Mythilin that allied with Hellcat Squadran against Apophis would continue to side with the Coalition and its allies in future conflicts. Their link to the Coalition would, in the early-mid 2340s, become Zena Lois, a former Oracle from Proterra, who was deemed "unworthy" by the priests at Proterra's Delphi of continuing as Oracle, but Hellcat Squadran (with Mythilin Intervention) convinced to let them take with them.