The Change is a transformation occuring among those members of the Tomol species living on Arethusa who have just reached adulthood, usually just past their seventeenth sun-turn, and not committed ritual suicide, leading to physiological and mental abilities far superior to those of standard humanoids, but also encompassing violent insanity.
The Change had always been a part of the evolutionary process among the Tomol. However, hundreds of thousands of years ago, the Shedai, remembered as the "Dark Gods" by the Tomol, found them on their original homeworld and attempted to use their powers as weapons in their struggles against enemies such as the Tkon. When the Tomol fought back, the Shedai tried to reign them in using careful genetic manipulation, but this plan backfired, as it was this tampering with the Tomol's genetic code that caused them to go insane. Eventually, the Shedai gave up and left the Tomol on their homeworld to destroy each other.
However, the Preservers, remembered as the "Shepherds" by the Tomol, transplanted a few hundred Tomol children to the second planet in the Nereus system, which their decendants would eventually call Arethusa, and installed a set of rules and rituals, including the Cleansing, to ensure that those Tomol exhibiting the first sign of the onset of the Change would commit suicide and therefore not endanger the rest of their people. A Preserver obelisk was also left behind to monitor the Tomol and act as a last defense in case one of the Tomol should go through the Change past the initial stage. In such a case, the obelisk would send out drones that would encase that individual and, if neccessary, everything else, in a sort of living rock.
Sometime before 731 BC, a group of Tomol refused to undergo the Cleansing and embraced the Change, but the obelisk eventually stopped them. Their unmoving figures came to be referred to as the "Endless" by the Tomol.
In 2269, the Tomol girl Nimur refused to undergo the Cleansing at the last moment and fled her village with her infant daughter Tahna, gradually undergoing the Change. After she discovered that the group of Klingons that had unexpectedly covered her escape wanted to use her for their own ends she killed all but one of them and set about recruiting her people to her cause and convincing them to embrace the Change, killing her village's priestess, Holy Sister Ysan in the process. She also confronted the crews of two Federation Starfleet starships present in the system, the USS Sagittarius and the USS Endeavour. Even though she was eventually turned into one of the Endless by the obelisk, her mate Kerlo continued the fight, having been reluctantly recruited by Nimur. However, the Starfleet crews were able to figure out the history of the Tomol and, thanks to their familiarity with the Shedai, develop a way to return the Tomol to their original evolutionary path, eliminating the insanity in the Changed. Once this treatment had been administered to all Tomol the living rock disappeared and the Tomol set about rebuilding their lives. Ysan's former protégé, Seta, told the Starfleet crews that they would be welcome to return and both groups parted ways. (Seekers novels: Second Nature, Point of Divergence)
The first sign of the onset of the Change is an unusual glow in the eyes of the affected individual. It is at this point that a date is set for this individual to undergo the Cleansing, overseen by their Holy Sister and guarded by the Wardens. Should the individual not do so, they will become steadily more resistant to physical damage, to the point where it would take several Federation Starfleet phaser rifles on their maximum setting firing at the same time to stop one of the Changed. Additionally, those among the Changed who are a little further along are able to transform their physical shapes at will. Aquatic adaptations, bird-like transformations and vaguely reptilian behemoths are among the changes observed. Those behemoths exhibit strength enough to punch through a Starfleet starship's outer hull.
Among the first of the mental powers to develop is the ability to perceive the auras around all things, including people. Inherent differences and sudden changes in such auras helps them to distinguish different species and individuals and detect mood changes or attempts at deception. This eventually develops into the ability to not just recognize the imminent onset of the Change in another Tomol, but to expedite it as well, essentially "turning" them. Powerful and precise telekinetic abilities also appear, enabling the Changed to immobilize entire groups of people at a time by lifting them off the ground and holding them there or either ripping their bodies apart entirely or merely breaking one bone at a time. Lastly, they develop the ability to communicate telepathically with each other.
It has been speculated that the Tomol's evolutionary path would one day take them beyond the need for physical bodies, but no such instances have been observed and no accounts have been found of this happening before. (Seekers novels: Second Nature, Point of Divergence)