The Fear of Katarn was a fundamental law of the universe which came into being with the emergence of Kyle Katarn immediately after the Big Bang, trumping even gravity, electromagnetism, and the nuclear forces. It was this operational guideline that led to the invention of the hyperdrive, when the universe broke its own laws to open up another dimension, hyperspace, where it was possible to break the speed of light.
The Fear of Katarn continued to exist in the deepest crevasses within the minds of all sentient beings from the moment sentience first emerged into the galaxy. When the Force was discovered by the Jedi and Sith alike, these monastic orders soon came upon this fundamental law of nature (the approximate moment is said to be about 0.003415 standard seconds after initial Force-awareness). Some of the first Sith Mega-Super-Overlords attempted to invoke the Fear of Katarn into a Force power that they could use to obliterate their enemies; however, all who attempted thus were immediately roundhouse-kicked into hyperspace by Katarn himself.
It was the discovery of the Fear of Katarn that led the Jedi Order into becoming strict adherents to the light side of the Force, not because of some immortal decree from Katarn, but because they were simply too terrified of what had happened to their Sith contemporaries when they attempted to harness the Fear.
Invoking the Fear of Katarn was a perilous business for any sentient being, no matter what their species, planet of origin, or alignment. It was only ever successfully invoked by Katarn himself, when he single-handedly overthrew Jacen Solo's Sith Empire by tossing the Anakin Solo into the Maw.