The Nacene were a highly-advanced sporocystian lifeform that originated outside the bounds of normal space, far beyond the Milky Way Galaxy. They were possessed advanced psychokinetic abilities which allowed them to manipulate matter and many types of energy.
While visiting the Milky Way in the distant past, the Nacene inadvertently caused severe devastation to the ecosphere of the Ocampa planet in the Delta Quadrant. One of their number remained behind to try to correct the damage, and became known to the Ocampa as the Caretaker. (VOY: "Caretaker")