Their faces were grotesque in appearance with members of this species being completely feral and savage as well as showed cannibalistic tendencies. A person transformed into a Fri'slen by sexual intercourse and a considerable exchange of bodily fluids. They were noted to live mostly underground and kept some victims alive in order to eat them, leaving nothing behind but bones. Though they might have appeared primitive in their actions, the Fri'slen were quite intelligent and cunning, allowing them to set traps for opponents.
One member of the species stated that they were born as a result of mutations and experiments and considered letting one victim live in order to allow the galaxy to know how they came to be. Furthermore, it was noted that they wished to simply survive, breed and thrive, which was their primary goals.
In 2135, the V'Shar dispatched a team of agents to deal with the Fri'slen threat. The team consisted of Denak, T'Pol, Eskren, Vekk'r, Ych'a and Yekda. At the end of the mission, only Denak, T'Pol and Ych'a survived. (ENT novel: Kobayashi Maru)