The powerful, non-humanoid species of the Caretaker and Suspiria, whose search for a successor stranded USS Voyager in the Delta Quadrant.
The Nacene is the name given in production and licensed material to the species of the Caretaker, a powerful entity encountered in the pilot of Star Trek: Voyager. On screen the species is never named. Its members are able to travel between galaxies and take on whatever form they choose, and their true appearance is a form of energy.
The Caretaker and a companion came to the Delta Quadrant from another galaxy. Their exploration accidentally devastated the atmosphere of the Ocampa homeworld, and in remorse the Caretaker remained to care for the surviving Ocampa underground. In 2371, near death, he pulls ships from across the galaxy, including Voyager and a Maquis vessel, in search of a compatible being to succeed him. He dies before he can return them, and Kathryn Janeway destroys his array so the Kazon cannot use it.
In Cold Fire, Voyager meets his companion Suspiria, who cares for a colony of Ocampa with expanded mental powers and blames the crew for the Caretaker's death. The species shaped the premise of the entire series.