The Ba'neth were an extremely xenophobic and reclusive Delta Quadrant species. They were known to have tentacles, and possessed a relatively large fleet of ships, which included at least one space station.
They inhabited the same region of space as the Kesat, who until 2376 regarded the Ba'neth as a myth, so little was known about them. Despite this, a Kesat investigator, Naroq, believed that they existed and spent much of his career searching for proof.
Through 2376 there had been twelve incidences of ships being attacked by unknown assailants in that region of space. No crew belonging to the ships attacked had ever survived until the Delta Flyer, a shuttlecraft belonging to the USS Voyager encountered a Ba'neth in that year. Lieutenant Commander Tuvok was assaulted when he interrupted it downloading tactical information from the Flyer's computer.
The species were found to utilize cloaking technology to help them remain undetected, and to observe other species traveling through their space. Captain Janeway sought the help of Naroq, who had devised a method of illuminating the cloak, to try to find the Ba'neth. Having modified Voyager's deflector dish, and knowing the cloaking frequency which Tuvok remembered from his scan, Janeway located a Ba'neth space station. They admitted to attacking the ships to assess the threat that they posed, and agreed to trade the photolitic converter used to find them for information that would help to treat the still-affected Tuvok. (VOY: "Riddles")