Rex Dacut was the third planet in the Firaz star system, located in Federation space in the Alpha or the Beta Quadrant. It had two moons. The world was quarantined as ships and visitors often mysteriously disappeared here.

During the Four Years War (reference stardates 1/94–1/98, c. 2250s), the planet was the location of a major space battle between Federation and Klingon Empire forces.

Reports showed that at least four Klingon ships had entered the Firaz system at one time or another; however, none were ever seen to leave. Orbital surveys made of Rex Dacut after the war found what might be two crash sites from the Klingon ships or, at least, their command pods, but there was no sign of any survivors. At least three landing parties, some Starfleet, attempted to survey and explore the planet's surface. Each disappeared without a trace. Sometimes, this happened so suddenly that voice transmissions and sensor telemetry cut off simultaneously. One party didn't last fifteen minutes before losing contact.

Starfleet placed a beacon marking the whole system as prohibited, forbidding approach for any reason, including emergency landings. Nevertheless, a ship was lost in the area at least once a year, likely containing prospectors seeking the resources described by old Orion records, and not checking the modern Starfleet ones.

It had a very hot tropical climate and harsh jungles, a terrestrial atmosphere and a 23-hour day. Orion records described its quality woods, petroleum deposits, and medicinal plants. It had a total surface area of 430,052,100 square kilometers, and with 23% land mass, a total land mass of 98,912,100 square kilometers. About 28% of its makeup was normal metals, 16% was radioactive elements, 9% was gemstones, and it had trace amounts of special minerals and industrial crystals (FASA RPG module: The Orions: Book of Common Knowledge).