The Eidelons were a pacifist race who inhabited the planet Arnessk 12,000 cycles before the Farscape storyline takes place. They possess a power best described as "the ability to influence peace". They have a very special gland located in their heads which grants them this ability. First, the gland acts as an empathic receptor, allowing the Eidelon to sense the wants and desires of those nearby, once the Eidelon has spent time acclimating to and understanding the creatures' psyche. Secondly, the organ vibrates to create an energy field which facilitates tranquility and rationality in those nearby. Using these powers, the ancient Eidelons could devise compromises that met the goals of the involved conflicting parties and could ensure that a peaceful solution to the conflict would be accepted.
As an Eidelon's peace-inducing power is limited to extreme close range, the Eidelons sought out a species to serve both as bodyguards and as enforcers of peace. The Eidelons took great care to choose a species that had yet to make interstellar contact. They found primitive humans, barely clothed, far removed on the galaxy's outer spiral. 27,000 years ago, they brought some of them back to their homeworld of Arnessk and employed genetic engineering to enhance them. This new species became known as the Sebaceans and they became the Eidelons' trusted acolytes. They took on the name of Peacekeepers due to their function.
12,000 cycles prior to the Farscape storyline, Arnessk became uninhabitable due to an attack by an undisclosed race using the Darnez Probes, and the Eidelons fled to other planets. Some remained in their Arnessk temple, however, and entered suspended animation of a sort, causing their temple to disappear from time. The Peacekeepers attempted to carry on once the Eidelons vanished. However, lacking the Eidelons' mediation abilities, they kept the peace with military force, and eventually became the militaristic Peacekeeper empire of contemporary times. When the three probes that comprised the Darnaz Triangle were reactivated by Crichton, Chiana and Jool, Arnessk became habitable again and the temple reappeared.
In Farscape: The Peacekeeper Wars, the last surviving Eidelon colony is discovered hidden on the water planet where Moya is healing. During the time they have been away from Arnessk, however, they lost their empathic abilities. Crichton takes a pair of the Eidelons to meet their ancestors in the hope they can be taught the power once more. However, the Scarrans appear to eradicate all of the ancient Eidelons so they cannot influence the empire. During a Peacekeeper-Scarran attack on Qujaga, Stark managed to give an Eidolon priestess the the empathy knowledge from an Eidolon named Hierachy. Crichton brought four Eidolons to Moya while Jothee used a Scarran and his transport to bring at least a hundred Eidolons. In space, the Eidolns watched as their world was torn apart by a universe consuming wormhole created by Crichton. When both sides agreed to a settlement and the wormhole dispersed, the Eidolons supervised the treaty on Moya.
The new Eidelon colony does eventually regain its powers of mediation through Stark, who gains the knowledge as he crosses over one of the dying ancestors, and he in turn passes the ability on to one of the colony. The event also leaves Stark's mind at peace, no longer filled with the pain of others, and his power to cross spirits over is gone.    
- Murphy, Kevin Andrew (2005), Farscape Forever!: Masks of Transformation, BenBella Books, Inc., pp. 211-218, http://books.google.com/books?id=PwELWeMMFIYC&pg=PA214&dq=&sig=FApitqK7mgf6quYK8XT0jP_wZ-M#PPA211,M1