Marayna's nebula was an astronomical object inside the Nekrit Expanse, an inversion nebula in the galaxy's Delta Quadrant. This nebula was artificially prevented from burning out with the help of a dampening field projected from a cloaked space station operated by Marayna. (VOY episode: "Alter Ego")
On stardate 40460.3, the Federation starship USS Voyager visited Marayna's nebula and found the unusual dampening field preventing the nebula's plasma strands from igniting the inversion nebula in a chain reaction.
In an effort to compel Voyager's chief of security, Tuvok, Marayna ignited specific plasma strands to damage the Federation ship. Tuvok convinced her to pass her lonely duty on to somebody else and return to the company of her people. (VOY episode: "Alter Ego")