A Federation colony planet with millions of inhabitants, chosen by the evil being Gorgan as the next world to conquer through the children of the Starnes expedition.
Marcos XII is the intended destination of the Enterprise in the Original Series episode And the Children Shall Lead. It is a settled Federation world, and the entity called the Gorgan sees its large population as the next step towards an army of followers.
The Gorgan has already driven the adults of the Starnes scientific expedition on Triacus to suicide and taken control of their children. Once the children are aboard the Enterprise he teaches them to use their powers of illusion on the crew, so that the helm is turned towards Marcos XII while the bridge officers believe they are still in orbit of Triacus.
Kirk breaks the spell by showing the children recordings of their dead parents. The Gorgan loses his hold on them and the ship never reaches the colony.