The Beta XII-A Entity - also known as (*) - was a non-corporeal entity who fed off strong negative emotions such as those caused by hatred and violence and the true main antagonist of the Star Trek episode. Day of the Dove.
Approximately one million years ago Q fell in with what he would later call a bad crowd, who used the Guardian of Forever to pull the Beta XII-A Entity into this galaxy. This bad crowd consisted of the Sha-Ka-Ree being, a being known as 0, and Gorgon. Among their many atrocities was the destruction of the T'kon Empire. The Q Continuum combined forces and defeated the group. During the battle the Beta-XII-A entity ran off and took shelter on Beta XII-A, where the crew of the Klingon ship IKS Kolode and the Federation ship Enterprise later encountered the entity.
After damaging the Kolode to the point that there were only a handful of survivors, the Enterprise took on the survivors and destroyed the Kolode due to the radiation coming from the ship. The Beta XII-A entity trapped a large part of the Enterprise crew behind emergency bulkheads, equalizing the numbers between the Klingon and Starfleet personnel on the Enterprise. It then tried to start an endless conflict between two groups, however when the Klingons and Starfleet personnel discovered that refusing to fight and good spirits would weaken the entity, the entity abandoned the Enterprise.
The Beta XII-A entity later encountered a Dominion ship bound for Terok Nor with codes to destroy the minefield preventing reinforcements from entering the Alpha Quadrant. The entity unwittingly saved the allies when it caused the ship's crew to kill each other, and became trapped forever in a spatial rift.