The Tomol were a humanoid civilization native to a planet located somewhere in the Milky Way galaxy's Alpha or Beta Quadrants.
History
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 naturally advanced 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 descendants 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 necessary, 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, encasing them in a kind of living rock that kept the individuals inside alive well into the 23rd century. Their unmoving figures came to be referred to as the "Endless" by the Tomol.
In the year 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 high priestess, 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)
Biology
The Tomol were humanoids with two sexes, male and female, green-skinned with pale yellow spots circling their ears and continuing down their backs. Their hair was silver and the eyes were usually gold, changing to red when gripped by the Change. They also had green blood.
When a Tomol reaches adulthood, usually just past their seventeenth sun-turn, they undergo the Change, a condition originally free of harmful side-effects but now accompanied by insanity. Because most Tomol went to their Cleansings after only seventeen or eighteen sun-turns while their offspring were still quite young or newly born, it was necessary to choose a pair of younger Tomol, typically around the age of ten to eleven sun-turns, to assume parental responsibilities for the offspring until they become old enough to tend to their own basic needs. Inevitably, when the Guardians were old enough, they produced offspring of their own - at which point their adopted charges often assumed the mantles of obligation as Guardians for some older Tomol's orphans. (Seekers novels: Second Nature, Point of Divergence)