In 2257 the Kelpiens of Kaminar went through vahar'ai together and freed themselves from the Ba'ul, who had controlled them for generations by culling those who began the process.
Saru had always been told that vahar'ai meant death, but when he went through it he survived and lost his fear. Returning to Kaminar, he learned that the Ba'ul had deceived his people to keep them docile, killing those who started to change.
The Ba'ul moved to wipe out the Kelpiens, but Discovery's crew turned the Ba'ul's own system against them and triggered vahar'ai in every Kelpien at once, leaving the Ba'ul unable to keep their grip. The two species began an uneasy coexistence, and the Kelpiens later joined the Federation.