The Klingon battle cruiser carrying Chancellor Gorkon to peace talks in 2293, where he was murdered by assassins in Starfleet uniforms.
Kronos One was the flagship of Klingon Chancellor Gorkon in Star Trek VI: The Undiscovered Country, a K't'inga-class battle cruiser. After the explosion of Praxis, Gorkon was travelling to Earth for peace talks, and the Enterprise-A met the ship to escort it.
After an awkward dinner aboard the Enterprise, two torpedoes appeared to be fired from the Enterprise and struck Kronos One, knocking out its artificial gravity. Two assassins in Starfleet uniforms and magnetic boots beamed aboard and shot Gorkon and others. The torpedoes had actually come from General Chang's cloaked Bird-of-Prey.
Kirk surrendered to prevent a battle, and he and McCoy beamed over to help. McCoy could not save Gorkon, and the two were arrested, tried for the murder and sent to Rura Penthe. Gorkon's daughter Azetbur took his place and carried the peace talks to Khitomer.