The Thing in the Well was the name given to a dark side entity that was spawned sometime before the Great Hyperspace War. It was created by the hatred of a single, unnamed Force-wielder who descended from disillusioned Jedi and Voss fleeing the ancient conflict on the latter's homeworld. Several generations after the Force-sensitive settlement was founded on Dantooine, the individual was born and rose to prominence, killing several of his neighbors before being sentenced to death.
Secretly buried alive in an act of vengeance, the individual's fury and hatred was such that it permeated the area, influencing the behavior of the settlers until – centuries later – the dark side's influence caused a short-lived civil war, further feeding the taint that poisoned the murderer's tomb and giving the undirected negative energy a conscious form, a living embodiment of the powerful feelings that had spawned it. Originally manifesting as illusions that lured unsuspecting villages to their deaths, the fear and panic caused by the ongoing disappearances empowered the entity further, allowing it to create illusions that could not only trick, but also kill.
Terrorizing the villagers before picking them off at night, the entity evolved further until the handful of survivors fled the mountain in search of a better life, leaving the hidden settlement abandoned and the entity starved of victims for millennia. Generations later, when Graultown was established in the nearby hills and began to expand, the entity started to target the new settlers, increasing its power more cautiously, until such a time it had enough power that even the more distant village of Purple Plains began to see the effect on the native wildlife and made the decision to evacuate to Khoonda, prompted by a distress beacon sent to them by the residents of Graultown in which they requsted Jedi help. Acknowledging the threat, the Jedi sent Jedi Masters Batauww and Hyin, along with their apprentices Avexiss Odé and Hessi Volann.
Unfortunately for the Jedi, they had underestimated the power of the entity, and after just one night Master Batauww temporarily lost his mind while Master Hyin was torn apart by villagers who had been twisted into bestial cannibals. While Hessi evacuated villagers to safety, Avexiss made her way further up the mountain after the entity spoke directly into her mind, inviting her to its domain in exchange for the unmolested retreat of Hessi and the villagers. En route to the mountaintop, Avexiss encountered the spirit of a young Zeltron boy, killed by the entity, which hardened her resolve and spurred her on to confront the evil specter. More spirits lined her path as she got closer, guiding her to the abandoned mine where she found a simple stone well. Avexiss knew the entity was beneath the water and, despite her fear, she dived in, falling through the liquid and emerging a few seconds later in an ancient chamber. There the entity tried to convince her to surrender her body, revealing that it needed a body strong enough to house its dark power without being consumed by it. Without one, it was bound to the burial site, unable to move until it accumulated enough power – something that would take millennia.
Avexiss refused and tried to physically strike down the entity, only to be swiftly overpowered and bound by a purple vapor that shattered her skeleton, allowing the entity to begin forcibly entering her mind. It was only when Avexiss accepted defeat that she was able to let go of her fear and fully accept the Force, achieving a state of oneness that allowed her to act as a conduit for the Force, repelling the entity and moving on broken bones to destroy it. The entity survived, however, as part of it had already taken root within Avexiss' mind, allowing it persevere, albeit in a far weakened state. Nonetheless, it remained a threat, growing steadily in power and waiting for the moment when it could seize control of Avexiss' body away from her.
When Ziost was consumed in the Sith Emperor's ritual, Avexiss was on the planet and seemingly doomed along with everyone else until the entity projected itself out of her body in an attempt to save itself, inadvertently taking Avexiss with it and propelling them both into the body of the teenage Togruta slave Natasha Butler, who had been evacuated to the orbital station above the planet. Once Ziost was destroyed, Avexiss returned to the surface to find her body and – after arguing with the entity – convinced her unwanted guest to project itself once again, this time into what remained of her own body. Capable of amazing feats of regeneration, the entity expended massive amounts of energy to rejuvenate the Pureblood's corpse, buying Avexiss more time to find a way to destroy it and leaving Butler free to leave, albeit changed by the intrusion into her mind.
Unfortunately, this sinister symbiosis gradually polluted Avexiss' mind, affecting her mood and mannerisms and slowly but surely grinding down her mental defenses over the decades that followed, even though the change was imperceptible to Avexiss herself for much of that time. Torem Odé, Avexiss' long-standing ally, experienced a vision several decades after the war between the Galactic Republic and Sith Empire which clearly showed that Avexiss would be consumed by the entity and her body would become the vessel for an evil on par with the late Sith Emperor. Torem's visions – a "gift" from the very same Voss ancestors whose home had birthed the entity – were infallible, but also showed her the path she had to take to avert the disaster. Steeling herself to betray her old friend, Torem waited until the moment she had seen in her vision, impaling Avexiss from behind and drawing the entity into her own body willingly, offering it a darker, more willing host to ensure it did not simply heal Avexiss' body. Before her former friends could apprehend her, Torem turned her newfound connection to the dark side on them, unleashing a wave of power that paralyzed them long enough for her to make her escape.
Torem served as the organic prison of for the entity for several thousand years until, during the Galactic Civil War, she was able to engineer events to fulfill the visions that guided her, destroying the entity with the help of the Jedi Knight Mahad and her own daughter, Jaesa Vakara, narrowly preventing it from falling under the command of Darth Sidious.