Bythos are a race of aeons who act as guardians of time or planar travel, intervening in places where tears between realities have developed, or correcting individuals who have abused planar travel.
Tasked with the maintenance of time and planar travel, bythos can theoretically appear on any plane of the Great Beyond, including the Material Plane on which Golarion is located. They are still rare in the extreme, and only exist in places where time itself is twisted, such as near large deposits of the skymetal horacalcum. They are unique among aeons for primarily inhabiting the Astral Plane; other species of aeon disperse across the multiverse after their creation. However, even they sometimes appear in Axis and the Boneyard.