The Beacon of Kahless was an ancient, sacred Klingon artifact. According to prophecy, it would call together the twenty-four Great Houses of the Klingon Empire, who would then unite in the face of a common threat. The individual who lit the Beacon was known as the Torchbearer.
The Beacon was approximately 150 meters long in non-activated form, with the appearance of carved stone encased in an ornate metal alloy exoskeleton. It emitted a scattering field that blocked sensor readings.
When activated, the Beacon extended a series of appendages on its hull, which emitted an immense photonic output, measured by the USS Shenzhou as over one billion lumens per square meter. So intense was the light that it was reported as a new star all over the quadrant. It also generated a powerful electromagnetic subspace waveform detectable over interstellar distances.
The Beacon was used as part of T'Kuvma's plot to reunite the Empire and trigger a war with the Federation. In 2256, the USS Shenzhou visited a binary star system at the edge of Federation space to investigate a damaged interstellar relay. While trying to determine what had happened, the Shenzhou discovered the Beacon hidden inside the system's accretion disk. Investigating the Beacon to identify its origins, Shenzou first officer Commander Michael Burnham was attacked by Rejac, the Torchbearer, which led to her accidentally killing him.
The revelation of a Klingon presence caused Starfleet to send reinforcements, which in turn led T'Kuvma to order Voq to light the Beacon. Moments after the Beacon's signal stopped, twenty-four ships carrying the Klingon High Council arrived in answer. (DIS: "The Vulcan Hello", "Battle at the Binary Stars")
In the Star Trek Online mission "Home", Ambassador Worf uses the Beacon of Kahless to summon the Klingon fleet to Empersa, the Founder home world in the Gamma Quadrant, to help assist the Alliance battle the overwhelming forces of the Hur'q.