Appearance and abilities
The entity appeared as a small, circular pattern of multiple swirling lights in different colors, although it became red in hue and emitted a high-pitched wail when "feeding". The entity fed on the emotions of hatred, anger, and primitive predatory instincts exhibited by humanoids in close proximity to it. If these emotions were not present, the entity was capable of creating conditions to spawn such feelings.
The entity could control the instantaneous transmutation of matter from one form to another, turning everyday objects into weapons and sealing bulkheads on 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 so that they remembered fictional family members and events that never existed. The entity appeared unable to communicate in any fashion understandable to a humanoid. Although its motives and nature were not wholly known, it lacked the same ethical standards of most humanoids and largely acted in a malignant fashion. Its only weakness appeared 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 the Enterprise. The entity then 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 soon found 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, confused and starved, fled into the depths of space. (TOS: "Day of the Dove")
The entity's special visual effects were designed by Mike Minor, on which he noted, "There was a little alien whirlagig that fed on anger. The original idea was to rig a puppet, but there wasn't enough time on the set, so I suggested an effect with a child's pinwheel that could be 'burned into' the footage in post-production. I put some jewels and little flashing sequins on it so it would sparkle and Van der Veer added some color effects. The shot was double printed by flopping the film so it wouldn't seem to spin in one direction." (Starlog, issue 25, p. 61)
The entity made an appearance in the Q Continuum book trilogy by Greg Cox, where Q referred to it as (*) – whether this unpronounceable glyph was its true name or if Q simply made it up to make the entity seem even more alien was never truly made clear. The novel states that the entity was the cause of a war that rendered Sargon's planet uninhabitable.
The entity also appeared in "Night of the Vulture", a short story also written by Cox, featured in Tales of the Dominion War. The entity delighted in the carnage of the Dominion War but unwittingly saved the Federation and Klingons when its machinations killed the crew of a Dominion ship en route to Terok Nor with the codes that would deactivate the minefield guarding the Bajoran wormhole. The entity then became permanently imprisoned in a spatial rift.