The Sisters of the Ninth Plane were responsible for operating a children's medcenter on the celestial body Novor during the High Republic Era. In the year 231 BBY, Ola Hest, who worked as an enforcer for the Hutt Clan, was dispatched to the medcenter to demand protection money; however, the Sisters refused to comply and ordered her to leave. Subsequently, Hest directed the thrusters of her frigate towards the medcenter, destroying it and causing the death of everyone inside. The enforcer later alleged that the event was unintentional, but she was eventually apprehended by Jedi Master Porter Engle.
While incarcerated on the Galactic Republic prison ship named Restitution in 231 BBY, the inmate Sestin Blin recounted the incident involving Hest at the children's medcenter to Nihil Tempest Runner Lourna Dee, who was also a prisoner on the Restitution. Blin also shared with Dee her conviction that the event was deliberate, suggesting that the enforcer was fully aware of her actions and intended for others to know her intentions.
The Sisters of the Ninth Plane received a mention in The High Republic: Tempest Runner, an audio drama from 2021 penned by Cavan Scott. It was released as part of Phase I of the multimedia project, Star Wars: The High Republic. The correct spelling of the group's name was later verified with the publication of the audio drama's script in 2022.