tentacles and teeth and eyes are good fun, but my personal favorite form of eldritch horror is eldritch geometry
fractals, infinity mirrors and paradoxical shapes all evoke this vague but unmistakeable sense of dread in the right contexts because they stand perfectly between the lines of the unknown and the uknowable, the familiar and the strange. you can recognize a fractal, or your reflection looking back at you multiplied infinitely between two mirrored surfaces, or a staircase that loops back on itself without a clearly defined beginning or end, but you can never truly understand it. you can look at it forever and try to comprehend what exactly it is you're seeing, but something will always escape you, like you're trying to hold water in your hands. the more you look, the more lost you get, and the harder it is to pull yourself back from the apparent eternity that stretches out before you. the support beams that hold up your sense of space and self are weakened by the impossibility of what you're seeing, leading to an unsettling sense of disorientation, and you're forced to confront the fragility of your own perceptions, the limits of your comprehension, and how easy they are to overwhelm. you are forced to face the fact that there are things which cannot be known and will drive you to madness if you try, not because the knowledge will destroy you, but because it can't be obtained in the first place.
cthulhu hasn't got shit on this
