Kudao was a planet located somewhere in the galaxy's Alpha or Beta Quadrants, in Federation space near the Federation-Klingon Neutral Zone that borders the Klingon Empire.
History and specifics[]
The Klingons attacked Kudao shortly before stardate 9521, in the year 2293, in a period of unrest along the border before with the collapse of the Klingon economy due to the destruction of Praxis. Many settlers were killed. (TOS novelization: The Undiscovered Country)
The 2293 attack was deemed to be a massacre by Tuvok and others. (VOY novelization: Flashback, ST novel: The Jackal's Trick)