The Nyrians were a race of humanoids who operated in the Delta Quadrant. Although externally similar to Humans, they were uncomfortable in the environmental conditions aboard Starfleet ships, preferring their environment to be warmer, and less humid. They also preferred their environments to be darker, to the extent that very bright light impeded their functioning.
The Nyrians did not consider themselves a cruel people, and went to great lengths to avoid warfare, but nonetheless acquired new ships, colonies and space stations without consent of their previous owners. Ships so acquired might be tasked with defending Nyrian acquisitions.
The Nyrian method was to use of their translocator technology to gradually exchange the original population with Nyrian agents. These Nyrians feigned innocence, then took control when they outnumbered the remaining crew. The victims were housed in a simulation of their native habitat on a biosphere vessel, and provided with food, water, medical supplies and entertainment from the victims' own cultural databases. The Nyrians considered their method to be more effective and painless compared to warfare. Groups imprisoned in this way were expected to accept their fate gracefully.
In 2373, the Nyrians attempted to take control of the USS Voyager using their translocator. The Nyrians gradually exchanged themselves for Voyager crewmembers at the rate of one every 9 minutes and 20 seconds, and put the Voyager crew in an Earth-like habitat on board their biosphere vessel.
At first the Nyrians made it appear as if they did not know what was going on; they acted just as confused as the Voyager crew upon arrival. By the time the crew realized what was occurring (with the exception of Captain Janeway who had been suspicious all along) the Nyrians outnumbered the crew and took control of the ship, with Chakotay- the last member of the crew to be displaced- only just managing to download The Doctor into his Mobile emitter and take the EMH with him before the ship was completely overrun.
Thanks to the aid of a resident of one of the other habitats, as well as B'Elanna Torres making modifications to The Doctor's visual sensors to help him identify potential portals in the environment, Janeway and Tuvok eventually gained control of the translocator device, and translocated two senior Nyrians into one of the coldest environments on the habitat ship. By threatening to transport the rest of the Nyrian crew to the habitat, they were then able to convince the Nyrians to surrender and set about freeing the thousands of other prisoners the Nyrians had taken.
The Nyrians invading Voyager claimed to be from a colony called Nyria III, in the Nyrian system. This system was unfamiliar to Voyager. One Nyrian claimed to be an astrophysicist with the title of doctor.