The Sounds of Thunder

Another of the mysterious signals leads Discovery to Saru's homeworld of Kaminar, where the fearful Kelpiens are preyed upon by predators called the Ba'ul who demand that Pike surrender Saru to them since Starfleet has agreed to stay out of the conflict between the two species. Pike refuses, but Saru turns himself over to prevent a fight. Tilly works with technologically-augmented Lieutenant Commander Airiam to sift through the Sphere's information on Kaminar: they learn that the fearless, post Vahar'ai Kelpiens were once Kaminar's dominant species and nearly eradicated the Ba'ul. The latter were only able to survive by using their superior technology to cull Kelpiens before they lose their threat ganglia and become fearless.

Summary from Wikipedia, CC BY-SA 4.0.