Hirogen training ship converted into a huge holodeck, whose holographic prey rebelled, killed their hunters and took the Doctor in Flesh and Blood.
The Hirogen holoship was a vessel the Hirogen had turned into one vast holodeck to train with holographic prey. They had acquired holographic technology from Voyager after The Killing Game and adjusted the holograms so they could feel pain and learn, which made each hunt more satisfying.
In Flesh and Blood, Voyager found the holoship with all but one of its crew dead. The holograms, mostly modeled on Alpha Quadrant species, had overcome their safeguards and turned on the hunters. Their leader, a Bajoran hologram named Iden, took the Doctor and B'Elanna Torres to help them build a home of their own.
The Doctor first sympathised with the holograms, but Iden grew violent and fanatical, and the Doctor ended up stopping him. The surviving holograms were allowed to keep the holoship and leave to seek a life of their own.