[A] nineteenth century British cult believed in [a] comet-god who sang to the stars and planets as it passed by them in its orbit. They said it destroyed those worlds it passed, by waking up demons or ancient gods ... who slept on each world.
—Kevin A. Ross, "The Music of the Spheres"
Ghroth (the Harbinger) resembles a small, rust-colored planet or moon with a single, gigantic red eye which it can close to avoid detection. Ghroth drifts throughout the universe singing its siren song, the Music of the Spheres. As it swings by a planet, any Great Old One or Outer God sleeping there is awakened by the song. This usually results in the extinction of all life on the planet or perhaps even the utter destruction of the planet itself.
Ghroth is believed to be responsible for the periodic mass extinctions that wiped out 90% of all life on earth, including the extinction of the dinosaurs at the end of the Cretaceous era. It may also have caused the destruction of the planet Shaggai, the homeworld of the intelligent, insect-like Shan.
Source: Howard Philip Lovecraft universe, created by Ramsey Campbell