Fenrahk is a giant black spider rahi living underneath Tiro. It is the largest of its species, that being the rahi "Krahka." The spider can live for long periods of time without any sustenance due to being able to store large quantities of nutrients inside its body. However, it does surfaces from its cavern from time to time to eat any smaller rahi or beings that happen to cross its path. Yet it remains undetected. This is because of its uncanny ability to camouflage itself so it blends in with the already dark caves, as well as the shadows projected from Tiro's massive trees.
Most Tiroans believe it to be a legend, just a scary story told to children to keep them from wandering off from their studies. Only a few unlucky beings know the truth, and most are no longer living. However, the candle head cult is aware of the creature, and they must occasionally lead lost travelers away from its domain. The leader of the cult, Vezon, is evidently a little bit crazy since he befriended the beast upon first meeting him.
Fenrahk could also work as one of the "Kal Emanates" as pitched by @Bokarda but for the purposes of this pitch he is intended to be a regular, albeit large, Rahi.