Dammar was the Nyrian who led the gradual takeover of Voyager by swapping the crew out one by one.
Dammar was a Nyrian, a species that captured ships, stations and colonies by gradually displacing their populations into comfortable artificial habitats. He was the first Nyrian to appear aboard Voyager, claiming he had no idea how he got there.
In Displaced, Dammar professed innocence while more crew members vanished and were replaced by Nyrians. Once only a few officers were left, he and his people seized the ship. The Voyager crew found themselves in an artificial habitat built for their comfort, but escaped, turned the displacement technology against the Nyrians and took back their ship.