The legend of Triacus stated that the planet was the homeworld for a band of marauders who were making constant war throughout the system of Epsilon Indi. After many centuries, these marauders were defeated by the very individuals who they preyed upon. The legend also mentioned that a powerful evil presence remained on Triacus, awaiting a catalyst to revive it, allowing it to spread its evil across the galaxy. This power was personified by Gorgan, the last of the planet's marauders.
In 2268, Triacus was the site of a scientific colony assembled by the Starnes Exploration Party, lead by Professor Starnes. On stardate 5038.3, one of the archeologists, Professor Wilkins, accidentally released Gorgan during an excavation of a cave near their encampment. It was noted that the destruction of whatever civilization that might have been there was produced by some natural catastrophe.
Gorgan influenced the children of the expedition and caused their parents to commit suicide. Gorgan made a further attempt, and was initially successful, to escape from Triacus aboard the USS Enterprise to spread its evil on to Marcos XII. (TOS: "And the Children Shall Lead")
In 2369, this planet was listed on the chart "USS Enterprise Personnel Database", which was displayed on a PADD used by Captain Jean-Luc Picard on aboard the Enterprise-D. (TNG: "The Chase", okudagram)