GROTH THE HARBINGER

[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

SHANS / INSECT FROM SHAGGAI

Planet of Origin: Shaggai , a world that orbits twin emerald suns where the Shan lived in globular dwellings in huge cities. As worshippers of the Outer God Azathoth, they erected pyramidal temples containg "multidimensional gates" whereby "that from Outside" (an aspect of Azathoth called Xada-Hgla) could enter. Shaggai was also the one ofthe titanic Baoht Z'uqqa-Mogg, Bringer of Pestilence, a Great Old One similar to a colossal scorpion with huge compound eyes interspersed with antennae, an ant-like, venom-dripping maw, and gigantic wings which was often accompanied by a swarm of stinging insects. Shaggai was destroyed about a thousand years ago.

At last a shape appeared, flapping above the ground on leathery wings. The thing which flew whirring toward me was followed by a train of others, wings slapping the air at incredible speed... I could ... make out many more details... Those huge lidless eyes which stared in hate at me, the jointed tendrils which seemed to twist from the head in cosmic rhythms, the ten legs, covered with black shining tentacles and folded into the pallid underbody, and the semi-circular ridged wings covered with triangular scales... I saw the threemouths of the thing move moistly, and then it was upon me.
—Ramsey Campbell, "The Insects from Shaggai"

The Insects from Shaggai, or Shan, are a species of sentient pigeon-sized insectoid who developed an advanced civilisation with some space faring capacities.

The brains of the Shan have six lobes, giving them the ability to follow three trains of thought simultaneously. Most Shan have a phobia of sunlight because the electromagnetic frequency of the Sun's rays poisons their metabolism. As a result of contact with Azathoth the Shan developed the ability of Kirlian Phasing, allowing them to pass into the skulls and brains of organic life and mindcontrol them and feed  off the electromagnetic impulses in the brain . 

Abut a thousand years ago, the Outer God Ghroth the Harbinger passed near Shaggai  and destroyed the planet. Only those Shan in their teleporting temples of Azathoth survived the catastrophe. The survivors teleported to their colony on the planet Xiclotl, where their brethren had enslaved the native inhabitants. 

It is believed that the Mi-go influenced the path of Ghroth to instigate the obliteration of Shaggai or even that Ghroth awakened the local Great Old One, The Worm that Gnaws in the Night, destroying the planet.

The Shan remained on Xiclotl for some time, but upon discovering the frightening nature of their slaves' singular religious practice, they teleported to the planet Thuggon. A horrific find on this world prompted the Shan to flee once more, this time to the planet L'gy'hx, aka Uranus. When this world proved unsuitable, a small band of the Shan teleported to Earth arriving in the Severn Valley region of England sometime in the Middle Ages.

On Earth, they usually meld with humans. Once ensconced within the cranium of a human victim, they use telepathy to gradually dominate and control their puppet. The relationship is completely parasitic. As long as it inhabits a human host, the Shan has some control over the host's actions, and the longer it is there, the more control it gains. It can, however, be driven out by trepanation.

Other than a few heretics, the Shan are divided into two factions: Fanatical worshippers of Azathoth's avatar Xada Hgla that wish to eliminate all other sects and consider other deities to be inferior or false, and a faction of amoral hedonists whose main purpose is to discover new experiences, most of which involve cruelty or depravity. Their sadistic fancy is often implemented through a host, preferably a sentient one. 

Source: Howard Philip Lovecraft universe, created by British author Ramsey Campbell, The Shan first appeared in Campbell's short story "The Insects From Shaggai" (1964).

Aniolowski, Scott D., et al. "Mysterious Manuscripts" in The Unspeakable Oath #3, John Tynes (ed.), Seattle, WA: Pagan Publishing, August 1991. Periodical (role-playing gamematerial). Online.

Campbell, Ramsey. Cold Print (1st ed.), New York, NY: Tom Doherty Associates, Inc., 1987. ISBN 0-8125-1660-5.

Carter, Lin. "Shaggai" (1971) in The Book of Eibon (1st ed.), Robert M. Price (ed.), Chaosium, Inc., 2002. ISBN 1-56882-129-8.

Harms, Daniel. The Encyclopedia Cthulhiana (2nd ed.), Chaosium, Inc., 1998. ISBN 1-56882-119-0.