Cosmic horror is the genre that deals with mankind's Primal and oldest fear which is the fear of the unknown. The distinction between the other Horror genres becomes clear when we compare it with traditional horror fiction and even religious elements which consist of Spirits, Vampires, Demons, Zombies and Man Made Machines of destruction while Cosmic Horror deals with the pure element of the unknown when compared to a zombie, demon or spirit we as readers are provided with information like the origin of the said horror creature and its purpose and how to kill it or just protect ourselves from it but in Cosmic Horror the information is scarce and more than that the information available makes an individual question his/ her position in the cosmic scale which causes an existential crisis not about the purpose in life but questioning the point of individualism when facing the creature from Cosmic Horror which dwarfs them as a species.
The word “Cosmic Horror” is swapped frequently with the word “Lovecraftian Horror” because of H. P. Lovecraft the man who popularised the genre of Cosmic horror through his works like The Call of Cthulhu, At the Mountains of Madness, Dagon etc
Image Source:Cthulhu
The Character Cthulhu shows up in H. P. Lovecraft’s Book “The Call of Cthulhu” Where Cthulhu is an extraterrestrial being who came down to earth from the stars and has titles like High Priest of the Great Old Ones, the sleeper of R'iyeh, the Great Dreamer and The Slumbering One. I would highly suggest you guys check out the Book yourself “The Call of Cthulhu” so without spoiling the story the core aspects of it is there is an Old God in monstrous proportion waiting under the depths of the ocean just sleeping and dreaming while waiting for the right time to wake up and once it does it would be the ultimate end of Humanity’s rule over the earth since there is no known way to kill and the horror entity in question is old as time and the mere knowledge of this monsters drive people to madness since for a monster of such scale and power to exist in the same reality as we do terrifies people as a species, not as individuals.
So, the key element of Cosmic Horror is the fear of the unknown, whether it involves known elements of nature or abstract concepts, some of which naturally cause fear or a sense of discomfort to people. For example, in the story “The Call of Cthulhu,” the fear of large bodies of water, AKA Thalassophobia, is used. While the Deep Ocean is a part of our planet, it still remains a mystery to mankind, pushing the fear of the unknown in it since we haven’t fully discovered the vast part of the ocean.
Image Source: Thalassophobia
So in principle, the unknown creates terror in our minds and hearts so much more than a source of fear that you can understand and dissect it as an idea which is near to impossible in case of cosmic horror for the ultimate rational conclusion is that a monster of Cosmic Horror is impossible to kill and the only option available to humanity as a whole is to give up and be at the mercy of the Cosmic Overlords who might not even think twice before killing the entire human race since we are nothing more mere ants in their eyes.
Image Source: Eldritch Horror
Apart from the element of the unknown and cosmic scale of things one more factor which makes this genre unique and more terrifying than the others is the “Sense of Powerlessness” which we as humans are forced to admit from time to time in our lives but how our modern society is built upon illusions in our day to day life to make us believe that we do have control way more than we actually do and so it is way harder for people, in general, to embrace the death of a close loved one or even prepare for their eventual end when old age and disease get to them. The desire to live a couple more days or months and the craving of the mind to make the flesh obey as it once did in their youth and when it doesn't people truly start to embrace the inevitable and acknowledge their “Sense of Powerlessness”
Image Source: Grim Reaper
A craving for horror as a genre stems from humanity’s need to listen to stories which give fear in their heart and a chill in their bones just as a means to escape from the mundane but when horror produces a cognitive dread that the mind can’t comprehend it’s sense of scale and limitlessness that is when the genre of “Cosmic Horror” is born.