This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Vahar'ai was the biological event unique to the Kelpien species. A painful process resulting in the loss of the threat ganglia and the apparent suppression of fear responses, Kelpiens going through it reached an evolved state. The vahar'ai was a crucial factor in the history of Kelpien-Ba'ul history. (DIS: "An Obol for Charon", "The Sound of Thunder")
Early symptoms of vahar'ai resembled those of a cold, but later the condition escalated with increased heart rate, spiking adrenal levels, increased neural activity, and pain that would render the average humanoid unconscious. Their signature symptom was the inflammation of the threat ganglia in a unique way. (DIS: "An Obol for Charon")
Around the 1st century BC, a significant portion of the Kelpiens were post-vahar'ai. Evidence suggested that these "evolved Kelpiens" were predators to the Ba'ul. During the 1st century, the number of un-evolved Kelpiens dropped to zero, bringing the Ba'ul to the brink of extinction. However, the Ba'ul bounded back, while the number of evolved Kelpiens dropped. It was theorized that the Ba'ul had used their technological advantage to lock the Kelpiens into subservience by enforcing the doctrine of the "Great Balance", allowing them to live while eliminating the threat they posed. As of the mid-23rd century, no Kelpien had experienced vahar'ai for over two thousand years. (DIS: "The Sound of Thunder")
By the 23rd century, Kelpiens believed that the vahar'ai was a signal when they were ready to be culled or harvested for slaughter. Described as their death process, the condition was believed to be terminal, either through culling or madness. (DIS: "An Obol for Charon")
According to Kelpien beliefs, to reach vahar'ai was a high honor, to be chosen by the "Watchful Eye" to maintain the "Great Balance." As a teenager, Saru asked his father Aradar what would happen if he reached vahar'ai by the next harvest, and Aradar explained the honor to him. (ST: "The Brightest Star")
In 2257, Commander Saru of Starfleet became the first Kelpien to pass its vahar'ai in over two thousand years. The offworld Kelpien's vahar'ai was triggered by sound waves from a communications attempt by a sphere lifeform. After continuing to serve in the crisis through great pain, Saru requested that Michael Burnham sever Saru's threat ganglia, which would result in his death before madness came. However, his ganglia fell off by themselves, and his vital signs returned to normal. Saru reported that the innate fear he had always felt was gone. Although glad that the vahar'ai was apparently not terminal, Saru was extremely troubled by what this meant for his people. (DIS: "An Obol for Charon", "The Sound of Thunder")
Shortly therafter, a red burst led Saru's ship, the USS Discovery, to the Kelpien and Ba'ul homeworld of Kaminar. There, the Ba'ul went to great lengths to neutralize the perceived threat of a Kelpien having reached vahar'ai, including the attempted genocide of the Kelpien species. However, actions from the Discovery led to all Kelpiens undergoing the event. (DIS: "The Sound of Thunder")