An inversion nebula was a type of nebula composed of unstable plasma strands. 24th century Federation astro-theorists theorized that they were so unstable they were supposed to burn out within a few years, but the phenomenon had never been actually seen in the Alpha Quadrant.
Because of this, in 2373 when the USS Voyager discovered an inversion nebula in the Delta Quadrant that was centuries old, it investigated how that could be. They found that an unusual dampening field was responsible for isolating each plasma strand as it ignited, averting a catastrophic chain reaction. The dampening field was generated from a hidden space station occupied by Marayna, whose function was to preserve the nebula so that its beauty could be enjoyed by her people.
Marayna surreptitiously tapped into Voyager's systems and interacted with the crew via a holographic character, during which she became fond of Tuvok. In an effort to compel him to stay with her, she used the nebula as a weapon, igniting specific plasma strands to damage Voyager. Tuvok eventually convinced her that she should pass on her duty to another and return to the company of her kind. (VOY: "Alter Ego")