Under-Dwellers were a gigantic species that inhabited subterranean caverns below the surface of Planet X, yet they yearned for freedom from their underground existence. Although they disliked light, they were capable of enduring it. These beings possessed a collection of black eyeballs above a set of jaws filled with pointed teeth.
During a mission aimed at obtaining eggs belonging to the Nameless species, individuals from the Path of the Open Hand cult found themselves under assault by one of these Under-Dwellers. This occurred after the cult members had gathered numerous eggs from the cave walls beneath the planet. Bokana Koss, a Force-sensitive Ovissian within the cult's group, utilized the Force to perceive the creature, asserting its connection to an impending blight. Koss cautioned that the Under-Dwellers sought to bind the Force and that their true desire was for the cult to leave the planet, thus initiating the blight. After impaling and devouring a team member named Galamal with its spiked tongue, the Under-Dweller pursued the group to the surface. There, it engaged in combat with another massive entity known as the protector, who was also trying to prevent the Path's departure from Planet X. However, Koss overcame the fear induced by his vision following the Under-Dweller's attack and gave his life to restrain the protector. By 228 BBY, the blight predicted by Koss was actively propagating throughout the galaxy.