Sanitarium

The Sanitarium was the only structure remaining on the surface of Nathema in the centuries after the Ritual of Nathema destroyed all life on the planet. The Sith Emperor Vitiate continued to the use the Sanitarium for his own purposes during the following centuries, building a secure vault that kept some of his most-guarded secrets. He experimented with training Force-users to survive the void in the Force, resulting in Nathema Zealots, who became guards of the Sanitarium, while day-to-day affairs were being taken care of by specially trained Abyssin Keepers. The powerful Force-users who opposed Vitiate were send to Sanitarium to be experimented upon, unable to access the Force to resist or escape. Eventually, Vitiate send his daughter Vaylin to the Sanitarium to subject her to mental conditioning, she became the only inmate to ever leave the Sanitarium alive.

Years later Vaylin assumed her father's Eternal Throne and became aware of her mental conditioning, ordering Jarak, the scientist responsible for it, to find a way to undo it. After Jarak had a breakthrough, he invited Vaylin to return to Nathema and subjected her to a ritual of his design. However, it did not go as intended and Vaylin's power raced out of control, killing everyone in its path. She eventually found the will to break her bonds and left the surface Nathema, after which she used the Force to remotely trigger the destruction of the Sanitarium.

History
After the Ritual of Nathema wiped out all life on the planet and left a void in the Force on Nathema, Vitiate located one of his secret vaults there, storing artifacts even he deemed to dangerous to use. This included a holocron that contained the essense of his father Dramath, whom Vitiate continued to torment over the following centuries. He also brought groups of Force-sensitives to the planet, forcing them to brave the Force void in an effort to create an army of incredibly powerful but utterly subservient Force users. However, his efforts were only partially successful; the final result was the Nathema Zealots, a group of Force users who survived contact with the void through a daily regime of rituals and meditation that focused their power, while also diminishing their free will. The disadvantage of such a process was that the Zealots became actually dependent on the void for their survival, becoming catatonic if they left the planet's surface. Nonetheless, Vitiate repurposed the Zealots as guards for the Sanitarium, build above the vault. Vitiate used the void left in the Force on Nathema to keep powerful Force-users who opposed him in the Sanitarium, where they could be experimented upon, unable to use the Force to escape or resist. Apart from Nathema Zealots, the Sanitarium was home to specially trained Abyssin Keepers, who kept watch over the facility's day-to-day order. The vault was guarded by native Voreclaws, while the halls were patrolled by mutated creatures from other worlds, who were enetically engineered to instill fear, terror, and obedience.

Around the time of the Treaty of Coruscant Vaylin, the daughter of Vitiate's alter-ego Valkorion, had shown both immense strength in the Force and an inability to control it. Seeing her as danger to others and a threat to his plans, Valkorion send Vaylin to the Sanitarium on Nathema to teach her obedience. Anomid scientist Jarak was given a laboratory in the Sanitarium and a free reign over Vaylin. Jarak subjected Vaylin to many horrors during her stay in the Sanitarium, as she was tested both physically and mentally in constant fear, pain and isolation. Eventually Jarak exchausted conventional methords and subjected Vaylin to torture by machines of his own design. In the end, Vaylin's mind was broken and while she herself became a cruel, vindictive person prone to temper tantrums, she remained unaware of the true purpose of her stay in the Sanitarium - being conditioned to become powerless against whoever spoke the phrase "Kneel before the Dragon of Zakuul" in her presense. Vaylin's mother Senya Tirall at one point attempted to rescue her daughter from the Sanitarium, but was stopped by Valkorion before she could take her daguhter offworld. Vaylin spend years in the Sanitarium, during which her brother Thexan was the only one to visit her and bring her gifts. Eventually, her other brother Arcann brought her back home to Zakuul, which remained his only visit to Nathema.

Years came by and Valkorion was eventually killed, his Eternal Throne assumed first by Arcann, and then by Vaylin. After a confrontation aboard the Gravestone made Vaylin aware of her conditioning, she contacted Jarak again and used her new authority as the Eternal Empress to order him to find a way to break her conditioning. Jarak returned to Sanitarium, where Vaylin provided him ample test subjects. Eventually Jarak informed Vaylin that he had made a breakthrough and Vaylin soon came to Nathema, where Jarak explained that the process to break her conditioning would be even more extreme, with nobody else surviving it so far. Unaware that the Alliance Commander had followed her to Nathema attempting to stop her, Vaylin agreed to the process. However soon after it started, it proved to be highly unstable, resulting in her power erupting through the Sanitarium. The Void corruption was impossible to contain, and spread like wildfire, draining and killing anyone it came into contact with. Even Valkorion, an incorporeal Force entity, was not immune to its effects. Jarak himself escaped, but was confronted by Lana Beniko and the Alliance Commander. Jarak brough the two to the Emperor's Vault, where before he was killed by a vault guardian, he gave them a helpful suggestion on how to restore the Vault's power, allowing them to eventually get out. While Beniko and Alliance Commander were restoring the power and investigating Dramath's holocron, Vaylin successfully broke her bonds and left the Sanitarium, unaware of their presense. Once in orbit, she used her newfound power to trigger to destruction of Sanitarium through the Force. Beniko and Commander barely managed to escape the Sanitarium right before the last structure on Nathema was destroyed.

Behind the scenes
Sanitarium first appeared in Betrayed cinematic trailer. It was visited again in Star Wars: The Old Republic: Knights of the Eternal Throne Chapter VII: Into the Void, which elaborated on its backstory.

Appearances

 * Betrayed