An ancient Klingon signal device aboard T'Kuvma's sarcophagus ship, lit in 2256 to call the twenty-four Great Houses to the binary stars and unite them against the Federation.
The Beacon of Kahless is a sacred relic said to date from the time of Kahless. When activated it sends out an intensely bright light and a signal across subspace that can be detected throughout Klingon space, a call the houses are bound by tradition to answer.
It features in the opening episodes of Star Trek: Discovery, The Vulcan Hello and Battle at the Binary Stars. After Michael Burnham kills the Klingon torchbearer on an ancient structure near a damaged Federation relay, T'Kuvma names Voq as the new torchbearer and has the beacon lit.
The leaders of the houses arrive to find the USS Shenzhou confronting his ship, and T'Kuvma uses the moment to provoke the battle that opens the Federation-Klingon War.