Once, there was a race of transcendent intellects that existed at the End of Time that protected the timeline from any changes, usually by inflicting non-existance onto anyone who dared manipulate it for any reason. They were known as the Terminals, and it was their duty to establish temporal stability.
At some unknown point however, a mysterious incident managed to remove them from the timeline through minute changes in history that may have prevented the Terminals from existing in the first place. Scholars believe the culprits were Lemurians (Having studied the absolute limitations of Skafoi) who then managed to place themselves as the new power in the cosmos, traversing time and reality itself before they were annihilated, though how true this theory is has never been determined. The Terminals' successors, the Guardians of Forever, are quite a bit more lenient, allowing the saving of loved ones, helping of those in the past, and other such things so long as one doesn't alter things too badly. Unsuccessful attempts at saving the Terminals from their temporal annihilation have been made every so often.