- You may be looking for the Battle of Cheron.
Cheron was a Class M planet located in the southern region of the Milky Way Galaxy near the Coalsack, which once supported a humanoid civilization of Cheron natives. In the mid-23rd century, this area of space was largely unexplored by Starfleet, so the race was previously unknown to the Federation. (TOS: "Let That Be Your Last Battlefield")