A powerful machine housed in a serpent-headed rock, worshipped and fed by the people of Gamma Trianguli VI, who it keeps in a timeless, childlike state.
Vaal is an ancient machine that controls the environment of Gamma Trianguli VI from within a great stone head shaped like a serpent. Its people, who call themselves the Feeders of Vaal, bring it fuel and obey its commands, and in return it keeps them healthy, long-lived and free from want, but without children, curiosity or change.
In The Apple, Vaal sees the Enterprise landing party as a threat, attacks them with the planet's dangers and pulls the ship toward destruction. Kirk orders the Enterprise to fire on Vaal until its power is exhausted, freeing its people to make their own choices.