The Beta XII-A entity is a Noncorporeal being composed of pure energy. Although first encountered on this world, it is unknown if it originated there.
The entity appears as a small, circular pattern of multiple swirling lights in different colors, although it becomes red in hue and emits a high pitched wail when "feeding". What the entity feeds on are the emotions of hatred, anger, and primitive predatorial instincts exhibited by humanoids in close proximity to it. If these emotions are not present, the entity is capable of creating conditions to spawn such feelings.
The entity can control the instantaneous transmutation of matter from one form to another, turning everyday objects into weapons and sealing bulkheads at a whim. It could pass through solid objects at will, teleport from one location to the next, and influence the thoughts of others. It could heal a humanoid’s mortal wounds, and could even alter a person's memories, to a degree that they remembered fictional family members and events that never even existed. The entity appears unable to communicate in any fashion understandable to a humanoid. Although its motives and nature are not wholly known, it certainly lacked the same ethical standards of most humanoids and largely acted in a malignant fashion. Its only weakness appears to be exposure to emotions of humor, good nature, and friendship; it was particularly susceptible to laughter.
In 2268, the entity created conditions by which a Federation starship, the USS Enterprise came into conflict with a Klingon vessel. The entity began fostering anger and hatred between the two crews, making the crew of the Enterprise believe the Klingons had wiped out a fictional Federation colony. The Klingons suffered an explosion on their ship which killed most of the crew, leading them to believe they had been the victims of a Federation sneak attack. Hence a great deal of enmity already existed between the two crews when the Klingon survivors were taken aboard ‘’Enterprise’’. Then the entity began work in earnest.
First the entity sealed bulkheads across the Enterprise, trapping many crewmen below decks so that the number of Klingon and Starfleet combatants could be equal. Then it sent the starship on a mad chase through the galaxy at maximum speed, with no way to change course or stop the ship. Finally, it created primitive weapons such as swords from everyday objects, to make the conflict between the combatants more brutal.
The outcome was predictable. The Klingons immediately attempted a savage takeover of the Enterprise, with Starfleet personnel fighting fiercely for their ship. Casualties on both sides would soon find themselves miraculously healed to fight again. One Starfleet crewman took to wandering the corridors in a murderous or lust-filled haze, assaulting the enemy at will in a misguided act of vengeance. Through all of this, the entity roamed the ship to feed on each act of hatred and violence.
It was only through a supreme struggle of will that members of the Starfleet crew were able to deduce the involvement of the "third party" entity and what it was doing. They realized it would be even more of a struggle to convince the Klingons to lay down their arms and join them in "laughing" the entity off the ship. Through careful convincing, however, the Klingons eventually did just that, and the entity fled into the depths of space. (TOS: "Day of the Dove")