The Arivne had exceptionally powerful psychokinetic and telepathic abilities. They manipulated a small black hole to function as a microstar to warm their rogue planet. They selectively disabled technology that caused them mental irritation. They could teleport themselves and others extremely long distances through space, materializing aboard vessels even at warp speed.
Without any of the five human senses, Arivne had a strong telepathic awareness. Fur covered, with four digits per limb, their hands and feet were identical, though not ape-like, with their limbs having extra knees and elbows. They had no head, just a furry lump. They had a mouth for breathing only, without any mechanism for speaking. They did not ingest or excrete. They absorbed nutrients directly through psychokinesis.
Nor did they have genitalia. New Arivne may be fashioned somehow via psychokinesis. Newly formed Arivne acted like tribal hominids, traveling with cudgels and spears, bows and arrows for defense from hostile animal life. They could also genetically alter other species of humanoids, turning them into hybrid Arivne.
Mature Arivne found themselves to be uncomfortable around those who had undisciplined methods of thinking and believed in natural laws of physics, such as Humans. They considered Human thinking noisily distracting.
Circa 2230, the Arivne were first contacted by another powerful psychokinetic and telepathic race, the Irapina, which were sending an invasion fleet of four Irapina starships containing 10 billion Irapina to occupy Federation space. The Irapina offered to leave the Arivne alone if they remained neutral during the upcoming conflict, and considered sending advanced scouts to Anomaly to study future opponents.
In 2269, Anomaly was discovered by the starship USS Enterprise. Realizing that Humans might be able to help them understand the Irapina, Arivne attempted to directly communicate through telepathy, but this was painful to both parties. Security officer Bill Hixon was subsequently transformed into a half-Arivne to facilitate communication, and he implied that he preferred his new form, as his new telepathic senses let him experience reality directly, rather than through the filter of physical sensations. Through Hixon, and by reliving personal traumatic events from six members of the landing party, the Arivne learned about decision making and deceit, realizing that Irapina promises were probably lies.
When the Irapina decided to send advanced scouts, the Arivne arranged to act as intermediaries and translators so that the Irapina could interact with Federation representatives within interactive, illusory tests. James T. Kirk, Spock and Leonard McCoy subsequently engaged the Irapina, with the Arivne secretly providing them with advice and later augmenting their physical strength to help them win.
After diverting the Irapina invasion fleet away from Anomaly and Federation space, the Arivne erased Anomaly's location from the Enterprise's computers and asked that Starfleet leave them alone. They believed that any meaningful contact between their peoples would not occur for millennia. (TOS novel: Planet of Judgment)