A planet where a scientific expedition dies under the influence of the Gorgan, an entity that seizes control of the colonists' children.
Triacus is the site of a scientific colony led by Professor Starnes. According to legend, it was once the base of a band of marauders, and in And the Children Shall Lead it is revealed to hold the Gorgan, a malevolent entity that has waited there to escape.
The Gorgan drove the adult colonists to suicide and gained control of their children. When the Enterprise takes the children aboard, the Gorgan comes with them and uses them to seize the ship, until Kirk breaks its hold.