Jiaasjen was a religious theory and ideal espoused by the Jensaarai movement, and originated with Larad Noon. The term was assumed to be a Sith phrase meaning "integrating the shadow", and referred to the concept of integrating the teachings of the dark side of the Force into usage of the light side. It was similar to Count Dooku's belief that a perfect being could draw on both sides of the Force equally.
History[]
Jiaasjen theory was developed by Larad Noon, one of the survivors of the Brotherhood of the Sith, who felt great guilt for his misdeeds during the Great Sith War. Noon developed the theory in an attempt to rationalize his wrongdoings, and to keep himself from going insane. He wrote extensively on Jiaasjen during his self-imposed exile on the planet Susevfi until his death.
Several thousand years later, during the Clone Wars, Jensaarai followers of the recently-deceased Nikkos Tyris led by a female Saarai-kaar arrived on Susevfi in search of Sith teachings. They happened upon Noon's writings on "integrating the shadow" and, with no other teacher to follow, the Saarai-kaar integrated his work into her own, resulting in a Force cult that was neither wholly light nor wholly dark in nature.
Sources[]
Evil Never Dies: The Sith Dynasties on Hyperspace (article) (content removed from StarWars.com; backup link)- The Complete Star Wars Encyclopedia