Necronomicon

 Most of whom read this will probably have heard the word on the title, Necronomicon, somewhere. It's the title of a fictional book, said to contain information on the vast horrors of the beyond, specifically focusing on a thing called Cthulhu.

This guy. (Wikimedia Commons)

 Who's that, you say? Well, Cthulhu is a Great Old One, those being entities usually able to travel through the vast void of space and sometimes that of time as well, all with a rather sickly obsessive-compulsive fetish about our own little Earth here. I mean, if there are countless other planets out there with countless other races on them to pester, why do most of these tentacled monstrosities seem oh so very stuck on our little rock? And why do all of them seem to have tentacles, or failing that pseudopods?

This whole lore was originally cultivated by a gentleman by the name of H.P. Lovecraft, a man seemingly somewhat obsessed with his Harder-Than-Average-English-Words-Thesaurus and horrors that cannot be described. He seems to have thrown a few gears down the road, but I recommend his horror stories: until he tries to explain all of his horror (oh, look, ancient space-faring aliens that are kind of like elves in being better than us in every single way) it's all rather good.

Of course, a plethora of writers have picked off where he dropped off and carried on with his work of what is nowadays known as the Cthulhu Mythos, named after the sinister squid-faced world eater allegedly sleeping in the Pacific Ocean, in a vast underwater city named R'lyeh (which also boggles the human mind. This is a bit of a theme). Even some of that is good, although personally "cheesy" is the word I tag to most of these scifi horror writers: if humanity is constantly plagued by vastly superior alien races bent on killing, enslaving or eating us and/or our planet, how the fuck are we still here?

This is the problem of most of laid-open "god" horror: you have an entity that wants to smack the lot of us off this piece of rock, and has the means to do it, in a multitude of ways. But for some reason he/she/it chooses not to. "It's an alien intellect so vastly beyond our comprehending that we could never understand it's motives or actions" only carries you so far before becoming a rather corny and not a very satisfactory explanation.