The Byzantium was a category four, Galaxy Class, Star Liner which the Doctor witnessed crashing alongside River Song and Amy Pond during the 51st century. It carried the last of the Weeping Angels and when the ship crashed, the Angel was let loose (DW: The Time of Angels). When River Song in her time line met the Tenth Doctor just before her death, she mentioned the Crash of Byzantium. (DW: Silence in the Library)
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The crashed Byzantium ship. (DW: The Time of Angels)