I was listening to Call of Cthulhu when I got inspired to make my own Elder Gods Mythos.
Since I a science geek, I can't not ignore that we evolved in this world. That's one of those reasons that gives me a hard time wrapping my mind around Traveller's Vilani and Zhodani. I've since devised other rationals why they exist.
I am currently working on a kitchensink setting, a post apocalyptic world that has elements of the Twilight Zone in it. I got inspired by Silent Hill's alien and disturbing flow of time and how ghost zones is great dressing to be ported into how bridges into another world would appear.
This dark sci-fi fantasy world, the Kitchen Sink setting, is where the elder gods come into the story. A creature that fit the profile of the elder beings had to have some kind of rationale (something that is disallowed in CoC; so please bear with me).
The Elder god Virus
What if these dark beings were a virus so old and evolved, they came from the universe before this one. What if they struck a perfect balance destruction and captive growth, and evolved to a level of sophistication that they became sentient, but as a virus they needed to propagate and destroy.
They are a virus that existed so long, that they only evolved as other sentient species evolved. They need to consume and destroy, and even evolved beyond mere physical trappings, but extended to information, time, and concepts I can't imagine.
These viruses are not just single entity but evolved into multiple competing entities. Competition and evolving into deeper and more complex organisms made them stronger and slowed their growth only to make them stronger and more voracious.
What if these viruses needed sentient creatures to take form. The virus has individualistic aspects that allowed it to have a unique bond with a creature, and it searches the universe for more unique mutations of biology, psychology and information. Every Probabilistic variance mattered to the virus, that it began to exist in other realities.
When the previous universe ended, so did the dominance of the elder gods but the virus bled into dark places where nothing could exist and nothingness protected it against annihilation.
In the new universe, this one, they have come back from the darkest depth of sleep. The virus has spread and transformed and created creatures of maddening aspects. This is where the fiends of these elder gods come from.
More scientifically most of these creatures feed on human misery, in an existential way that is very difficult to grasp. When they consume beings, they consume them in their mind and in all the probabilistic states of existence they can have. Sentient creatures, because of their sophistication in how data, time, and biology interact attract the virus and feed its creatures.
The Mind Feeders
There is a creature that feeds on minds, impregnating a being with their own viral loaded enzymes and increasing their cognitive activity. They slowly consume a persons mind, with their tentacled kiss as the mind goes overdrive. The virus causes the brain to improve its plasticity, and begin to slowly move around its functions in order to keep the mind alive until every morsel is consumed.
The virus has enslaved a psychically talented race, and shaped it to meet its programmed desires.
The Blood Feeders
There is another creature that feeds off the brain through the blood. Its own virus loaded enzymes fills its prey's blood with euphoria, and a hormonal and neurological cocktail. This creature prefers to keep the prey alive as long as possible, but it burns out its prey's mind emotionally and physically. This virus can take many shapes and many hosts. Once taking on a host, everything beneath the skin melds into a homogeneous muscle-like system that has organ-plasticity (meaning it can easily adapt any part of itself for any specialized role) making it almost impervious to any injury. It has an adaptive programming in its design, that it takes the host's genes and biomorphicaly enhances the host's appearance to be at it's sweetspot. Its only vital organ is the brain of the host it has occupied, as it becomes the main digestive and energy storing organ of the virus. Its neurological functions has also plasticity.
Creatures of the Elder gods
The more primitive and unsophisticated viruses and hosts are merely tyrants and slavers. They are given hyper aggression, along with improved cognitive functions, and metabolic control. They provoke the most destructive and irrational side of its host, feeding off the pain, madness, agony and regret of its host.
Psionics and Magic.
The virus's multi-reality and space-time aspect makes Psionic and Magical minds a signal light in the darkness. Since the virus seeks to exist in all manner of reality, and Psionics and Magic tap these same spaces - this become a beacon to the virus.
Another Creature of the Elder Gods.
The other creature of the Elder Gods are those who think they can tame, control or even harness the virus. what falls in this category are the Psychics and Mages, who replicate the sophisticated feats of the virus and bend space and time. There are also demons, powerful forms of the virus that are not exactly physical in nature, they can be understood and tamed to an extent...
Some Insight on the Virus
The virus milks mirror neurons for the data feedback loop they can create, and all the empathic abilities of a sentient mind. The virus is weaker and less sophisticated in creatures that have a limit in these cognitive abilities. Social Animals are great food stock for the virus; the more neurological empathy load to weight of the animal, the more valuable to the virus.
The kitchen Sink setting.
GURPS Kitchen Sink is a Gdoc I'm working on, and in my previous computer I saved a Digest size (half of a letter sized paper) Random Character Generation Pamphlet. So its inspired by CoC and a lot of dark settings. You can choose to fight them, but since these creatures are not game balanced, they really will TPK if encountered... the way a party should not pick a fight with a dragon.