A Delta Quadrant species that captures ships and worlds by secretly swapping their inhabitants, one at a time, into prison habitats.
The Nyrians are a humanoid people of the Delta Quadrant who expand their territory without open war. They use a transport technology to swap members of a target crew with Nyrians one by one, until the vessel or planet is entirely in their hands. The displaced people are kept in a large complex of environmental habitats suited to their own needs.
In the episode Displaced, Voyager's crew began vanishing in this way and woke up inside the Nyrian habitat, while Nyrians took over the ship. Captain Janeway and B'Elanna Torres led an escape through the complex, met prisoners from many other species, and gained control of the transfer system to retake Voyager.