Trilan was a planet that supported a Vulcan colony. Some of Vulcan history's most famous models of Vulcan lyrette harps were crafted on Trilan by Separ. (TOS reference: Star Fleet Technical Manual)
In 2135, a race of mutated Vulcans called Fri'slen plagued the outpost on Trilan, murdering, cannibalizing, and spreading their infection. That year, a team of V'Shar agents, led by Denak, pursued these beings into their underground dwellings. Three of Denak's team – Eskren, Vekk'r and Yekda – were killed during the mission, while Denak and two others – T'Pol and Ych'a – survived. The caverns were later firebombed to eradicate these mutants. (ENT novel: Kobayashi Maru)