Quat Nevas is the main source and antagonist behind all the events occurring in the series as well as Mithra's true master. It is a planet eating entity which existed long before humans were born, as stated by Mithra.
It was originally thought to be an asteroid, named Orouborous, on its way towards earth. In Psyren it is what causes the world's barren landscape, starting with "The Day of Rebirth". Asuka Yoshina described it as an asteroid seemingly with a mind of its own. Orouborous zigzags through outer space in a snake pattern producing its own light that flashes on and off like a signal.
Mithra is in communication with it. As a planet-eating life form it has been eating planets long before humans were born. Every time it would weaken, it would eat a living planet for its reincarnation and the planet would become part of it. Quat Nevas didn't have enough energy to break into Earth's core to devour it, so it had Mithra collect energy to which she used Miroku to get. When Quat Nevas tries to devour the Earth Amagi and Grana sacrifice their lives to push it into space and away from Earth, thus saving the planet.