The Dark Sisters were a group of female nest-maid blind snakes who were masters of potions and poultices, living in a time only a few centuries before Soren was born. They resided in the hollow of an oak tree formerly inhabited by a family of Great Gray Owls on the border of the Forest of Tyto.
During one particularly damp period in the season of the Golden Rain, they were visited by Elva the female Elf Owl and Sanders the male Barn Owl (members of the Ga'Hoolology chaw) who sought a cure for The Great Ga'Hoole Tree since it was recently suffering from blackfire mushrooms - however, the owls were not sure what the cause of the illness was at the moment. The Dark Sisters said they could give them what they sought, if the owls knew what they sought. Elva and Sanders tried to explain the growing streaks in the roots of the Great Ga'Hoole Tree, but without knowing the name of the condition at first, the Sisters could not concoct a cure.
Soon the Dark Sisters were called upon once again by the Ga'Hoolology chaw, which by now had the information needed (thanks to Aude the old male tree healer Snowy Owl from the Shadow Forest). The Sisters chanted that Lyssop was what was needed. With that, they slithered from their hollow onto the forest floor. They told the six owls to return in two days. When they returned, they were given a bottle of bright green liquid. The Sisters instructed that the owls must keep the afflicted roots moist with this liquid. Two weeks, and the herb should do its work. Isolde the female Burrowing Owl (the famed Ga'Hoolologist and ryb at the time) gave the Dark Sisters her gratitude.
With the help of the Dark Sisters, black fire was eventually eliminated from all the Southern Kingdoms.