Troyius was an inhabited planet, the outer of the two, the other being Elas, found within the Tellun system in the Alpha Quadrant. This was the homeworld of the Troyians, a spacefaring humanoid species.
In 2256, this planet was mentioned with the star system's name on the star chart "Alpha/Beta Quadrant Overview" in the ready room aboard the USS Discovery. (DIS: "Magic to Make the Sanest Man Go Mad")
The planet had rich deposits of dilithium ("radan" in the Troyian language). In 2268, both planets possessed the capability of mutual destruction. The wedding of the Dohlman of Elas, Elaan, to the ruler of Troyius preserved the peace. (TOS: "Elaan of Troyius")
In 2399, this planet was mentioned with the star system's name on a Federation star chart that was in Fleet Admiral Kirsten Clancy's office at Starfleet Headquarters. The system was in or near Federation space. (PIC: "Maps and Legends")