A guide to Shadow of the Comet
Poor Carter...So young...And already in the next world -
Randolph Carter was a writer in Boston, who used to investigate into cultural beliefs and practices of Far Eastern, Middle Eastern, and pagan European peoples, and inquiring into the ideas delved by them. He was a secular atheist, and he believed that this trait gave him a more fanciful mind than religious people, who don't like to entertain supernatural possibilities beyond what their scriptures tell them. Two scarring incidents dote his life, once when his friend Warren disappeared in the caverns beneath a swamp, and he woke up with a complete blank in his mind the next day out of sheer terror of what may have happened, and when his schoolteacher friend nearly died after they were gored by some seemingly disembodied animal with horns on its head. Out of the trauma of these incidents, Carter felt that there was some semblance of dream in this world, that there is a transcendental multiverse of which the material world is one illusion that hides many things that exist outside it. Carter descent into dream and fantasy as he starts understanding them take a strong form when he starts believing, or even becoming, a reporter called John Parker, and finds himself dreaming things ahead of himself in advance, as if he existed in many possible worlds at the same time. When his dreams as Parker see Parker killed, he is killed, by the sheer shock of the mind, and he is indeed "already in another world". (see: the Lone Statement of Randolph Carter, the Unnamable, Dream Quest of the Unknown Kadath)
Heart attack -
In stories like From Beyond, when people know how some people can manipulate the fabric of this universe by doing things that exist beyond this space-time continuum, and know that they are not safe anywhere from things and creatures from beyond who are always a heartbeat away from disintegrating them, they actually die or come to clinical exhaustion from the sheer horror of it, because they'd rather shut themselves down quickly than face cosmic horrors that men like Narackamous (or his masters) can unleash.
Similarly, carrying the Necronomicon disintegrates Parker, because creatures always floating across the fibre of space and time don't pay attention to people who are not bothering with it, but will gladly rush in and tear the man out who enters into it and sees their presence. It is the horror of this disintegration that killed the antagonist from the above mentioned story.
Hambletons -
Impregnation with an ancient monster to conceive monstrous twins is an idea borrowed from Lovecraft story The Dunwich Horror, where a crustacean creature with long ropy arms, Yog Sothoth, impregnates a woman, and leads to two monstrous twins being born. Here, they replace Yog-Sothoth with Dagon, which results in inconspicuous mutations only half as monstrous.
Gypsies -
In Lovecraft stories like Horror at Red Hook, gypsies and other mystical eastern originated peoples are the last survivors of tales and practices from the "pre-Aryan" world, and these practices relate to the vestiges of knowledge of the ancients left with certain early human civilizations. Their rituals help invoke entry into those parts of the multiverse where one communes beyond space-and-time with people and creatures across time.
Boleskine -
Boleskine's ability to communicate to John Parker from beyond time long after he is dead, comes from the story the Shadow Out of Time, where a race of ancient creatures preserve themselves by having their mental frame disembody from themselves and reach out to creatures and species in the future, because the plane of the mind exists outside the material world.
Constellations -
In stories like Polaris, the position of stars and objects in the sky brings out connections and memories of things that happened a long time ago, and with the comet that passes in the sky every 76 years, it leaves its "shadow" of the terrible things that happened in the past.
Mic Macs -
The Mic Macs are known as They Who Worship The Night Howler, and interestingly, cults of people in parts of England who worship the Night Howler raise human cattle to feed on them as food in honour of the Howler, as seen in The Rats in the Walls. This affords them special powers, as it did to the Mic Macs.
I'll try to detect more references and inspirations from Lovecraft stories when I have more free time. :D