The Harimari Ascension was one of the events that coincided with the Day of Ashen Skies in the Year 1 in which the Harimari tiger-people were born. Most of its details are lost to history and engulfed by myths, aside from that date (though records of harimari only begin in 2 AA, so some scholars claim it may have actually happened a year or two prior to the Day of Ashen Skies, or in the direct aftermath thereof.)
The harimari themselves, along with many adherents to High Philosophy, attribute the Ascension to the legendary "Eye of Ascension", through which their tiger progenitors passed and emerged as sentient beings. That said, the logistics of such a process are dubious, and no such artifact has been recorded in any of the Order of Chroniclers' expeditions, nor do the harimari themselves claim to have access to such a portal. That said, given the volume of High Temples in the region, and their incalculable depths, it is impossible to fully rule out such an idea.
A theory more popular in recent history also points to the High Temples, and their strong spirit-manipulating powers. It argues that the Day of Ashen Skies must have caused some sort of malfunction in Raheni temples, where instead of suppressing the local spirits of land and sky, they were channeled. For whatever reason, their receptacle was the Ascension's Peninsula tiger population, and the product of being imbued with those quasi-sentient beings' essence resulted in the harimari. The knowledge transferred in this process, it is argued, explains why the harimari did not behave with total bewilderment upon transformation from mere jungle beast. It may also explain, at least in part, why Rahen does not suffer quite the same level of spirit presence as eastern Haless.