- You may be looking for the homonymous species.
Trion was the home planet of the humanoid Trions.
History
Trion had been colonised by the Time Lords millions of years ago. (PROSE: The Quantum Archangel)
Early in its history, Trion was attacked by the Tractators, an experience so traumatic to the Trions that it survived as an ancestral memory for millennia. (TV: Frontios)
By the 20th century, Trion was an advanced world with many colonies, some of which were centuries old. (PROSE: Lords of the Storm) They had spacefaring and transmat capabilities, and were on the cusp of developing time travel. (PROSE: Turlough and the Earthlink Dilemma)
At around this time, Trion suffered a civil war. At least some of those on the losing side were exiled rather than killed; Vislor Turlough was among these. Even young children, such as Malkon Turlough, faced exile; it seems to have been applied to rebellious families rather than individual combatants. The exile was lifted by 1984, and exiles were permitted to return. (TV: Planet of Fire)
The Tenth Doctor and Emily Winter visited Trion to make sure that Turlough didn't write the diary that the Advocate was using to manipulate Matthew Finnegan. (COMIC: Final Sacrifice)
Geography, flora, and fauna
Trion was located in a binary system; the Trions called their suns Minor and Major. It had three moons: Cu Chulainn, Cu Roi, and Njordr Nerthus. Njordr Nerthus had an atmosphere, but was not livable.
One Trion continent, Slotisland, was reserved for Trion's other native species, the Slots. Slotisland was home to the Slotsruins, traces of the vanished civilisation of the Laima, but the Slots kept the Slotsruins behind a force field to keep it secure from outsiders. Slotisland's secluded location, in the middle of an ocean that covered most of Trion's eastern hemisphere, was undoubtedly helpful in this regard. (PROSE: Turlough and the Earthlink Dilemma)
The main Trion continent, home to the Trions, was divided into several regions, among them Ohwrotco, Noved, Dnalvelc, Norring, and Tronna. An impossibly vast chasm called the Giant's Drop was a popular tourist attraction. (PROSE: Turlough and the Earthlink Dilemma)
Trion was also home to the Mobile Castle, a bizarre and ancient artefact poorly understood even by the Trions themselves. (PROSE: Turlough and the Earthlink Dilemma)
Vislor Turlough once described the trees on Trion as being three times the size of Earth trees, with plate-like thick mauve and purple leaves growing in spirals up blood-red trunks. (AUDIO: Loups-Garoux)
As Trion had vulcanologists, (TV: Planet of Fire) it was almost certainly a volcanically active world.