Joiners are non-Killik individuals who have been absorbed into the Killik hive mind. Prolonged exposure to the powerful pheromones released by Killiks causes an alteration of the subjects' brain chemistry, making them subservient to the nest's desires. Long-term exposure can turn a subject's eyes completely black, though this appears to be the extent of visible physiological changes.
Initially, Joiners retain many characteristics of their own personality, as well as some sense of individual identity. At this point Joiners often describe the relationship as symbiotic, feeling they gain more from the Joining than the Killiks. Over time, however, their personalities become more and more subsumed, until they cannot conceive of any existence away from the hive mind.
In turn, a nest also absorbs the knowledge, skills and personality traits of its Joiners. A single Joiner rarely changes a nest's nature, but a nest that Joins an army of warriors may become war-like, and a nest that Joins a party of explorers may gain a degree of curiosity and wanderlust.
It is possible to medically reverse the Joining process even in the later stages. However, most Joiners‒even those experiencing the initial transformation‒refuse to willingly submit to the procedure.