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The Ba'ul are a hostile species that was introduced in the Star TrekShort Treks episode "The Brightest Star".
Technologically advanced, the Ba'ul shared their homeworld with the more primitive Kelpians, and preyed upon Kelpians. The Ba'ul demanded that the Kelpians sacrificed themselves upon reaching vahar'ai - leading the Kelpians to believe that it was a terminal condition and that they had to ritually sacrifice themselves to the Ba'ul pylons to maintain the Great Balance.
In fact vahar'ai was the natural maturation process for Kelpians. Several thousand years ago it was routine for Kelpians to survive vahar'ai. Unfortunately such Kelpians preyed upon the Ba'ul. Eventually the Ba'ul retaliated and wiped out any post-vahar'ai Kelpians.
For the next 2,300 years the Ba'ul killed any Kelpians that had reached vahar'ai by forcing Kelpian priests to lead such Kelpians to be ritually vaporized by the Ba'ul pylons. The Ba'ul manipulated the Kelpians into believing that the sacrifices were necessary to maintain the Great Balance that kept Kaminar's fragile ecosystem intact. Interacting with Kelpians solely through their technology, no Kelpians had seen Ba'ul in thousands of years prior to 2257 when one revealed itself to Saru.
In 2257 the Kelpians finally learned the truth after Commander Saru of the USS Discovery survived vahar'ai and returned home to Kaminar to investigate a sighting of a "Red Angel" that had appeared in a number of places throughout the galaxy. The Discovery crew was able to trigger the Kelpians to undergo vahar'ai en masse.
Rather than allow the Kelpians to mature naturally, the Ba'ul decided to commit genocide against the Kelpians. Before the Ba'ul could kill the Kelpians their weapons were deactivated by the Red Angel.
Afterwards the Kelpians dedicated themselves to figuring out a new balance between themselves and the Ba'ul, one that was less exploitative to the Kelpians than before.