History:
Alien life-form of unknoen origin, composed of pure energy, that thrived on the energy of negative emotions. The entity, first encountered at planet Beta XII-A, was capable of manipulation matter and the minds of its victims. It created a confrontation between crew members of a Klingon ship and the Starship Enterprise in 2268. Pitting these longtime enemies against each others, the entity fed on their anger, growing stronger as their hatred increased. The entity was defeated by a powerful collaboration between the Klingons and the Enterprise crew.
TOS "Day of the Dove"
Note that the entity was not given a name in the episode, and we're not even sure it came from planet Beta XII-A. The entity's special visual effects were designed by Mike Minor.
Star Trek Encyclopedia II
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