
Slithery D |

If you want them to become an actual demon, that's pretty much impossible. If you want them to become a monstrous thing from the mythos, plant form into a mi-go, or take monstrous physique into a deep one.
There are rules for becoming a demon in Book of the Damned Volume 2. It takes the demonic obedience feat and a lot of very bad stuff over a very long time with some intermediate steps.

Baval |
Scorned Heart Witch from Into the Breach: The Witch by Flying Pincusion games. Gets its spellcasting 4 levels late, similar to a paladin, in exchange gets rage like a barbarian, but instead of raging gains a "Vengeance Pool" you can use to apply Summoners Eidelon Evolutions to yourself, plus attack damage and armor bonuses.
Or, any Sphere Magic class from Spheres of Power by Drop Dead Studios with the Alteration sphere, choosing the appropriate talents to transform yourself to gain any powers you like.

Lastoutkast |

If you want them to become an actual demon, that's pretty much impossible. If you want them to become a monstrous thing from the mythos, plant form into a mi-go, or take monstrous physique into a deep one.
Sorry I wasn't clear in my post. Imbicatus has closer to what I'm looking for. I'm trying to figure out a class build that lets me become a harold of cthulhu. I'm thinking mini cthulhu.

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You could use Alchemist as a starting point, take tentacle a couple of times, perhaps wings. Also, be a qlippoth-spawn tiefling if possible.

QuidEst |
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While not Cthulhu-themed exactly, the best horrifying thing I can think of is Aberrant Bloodrager + Aberrant Tumor + Aberrant Bloodline Familiar (raccoon, Mauler archetype). A portion of your body can detach as a cat-sized mass of flesh, claws, and teeth, capable of squeezing through fine cracks and gaps and growing to the size of a person.