An advanced alien race decided to share its knowledge of atomic power with the less developed Proscilican planet, hoping they will use the knowledge to advance their own civilisation. However some of the Prosilicans decided to use their new found knowledge to create nuclear weapons instead. This inevitably led to wars between different factions and eventually the whole planet was destroyed in a nuclear holocaust. When the advanced alien race that had given the Prosilicans knowledge of atomic energy returned to the planet and saw the devastation they had caused, they took an oath to never interfere in the matters of others again and only to record their deeds hereon. Thus was created The Watchers oath.
The Prosilicans were originally created by Stan Lee and Larry Lieber in Tales of Suspense #53 (1964). In 1968 Gene Colan re-imagined them in a reprinting of The Watchers origin in The Silver Surfer #1.