I don't feel qualified on the Chtulu mythos enough to write definitive stat blocks, but let's take a look at some of what we have:
First Up: Deep One
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The Deep Ones are a race of intelligent ocean-dwelling creatures, approximately human-shaped but with a fishy, froggy appearance. They regularly mate with humans along the coast, creating societies of hybrids.
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Lovecraft describes the Deep Ones as a race of undersea-dwelling humanoids whose preferred habitat is deep in the ocean (hence their name). However, despite being primarily marine creatures, they can come to the surface, and can survive on land for extended periods of time. All Deep Ones are immortal; none die except by accident or violence. They are said to serve the beings known as Father Dagon and Mother Hydra, as well as Cthulhu. They are opposed by mysterious beings known as the Old Gods, whose powerful magic can keep them in check. This detail is one of the vestigial hints that August Derleth developed as the mostly unnamed Elder Gods.
Okay, so we have immortal aquatic humanoids which can also breath air. Seems to me that they could use the statistics for standard Atlanteans with the addition of the Immortality power, and just describe them as looking different.
Next: Shoggoth
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It was a terrible, indescribable thing vaster than any subway train—a shapeless congeries of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and un-forming as pustules of greenish light all over the tunnel-filling front that bore down upon us, crushing the frantic penguins and slithering over the glistening floor that it and its kind had swept so evilly free of all litter.
— H. P. Lovecraft, At The Mountains of Madness
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The definitive description of shoggoths comes from the above-quoted story. In it, Lovecraft writes about them as massive amoeba-like creatures made out of iridescent black slime, with multiple eyes "floating" on the surface. They are described as "protoplasmic", lacking any default body shape and instead being able to form limbs and organs at will. An average shoggoth measured fifteen feet across when a sphere, though the story mentions ones of much greater size.
Okay now these have a basic blob-like form with a standard size, but can have permanent Growth power applied to them to represent larger versions. The bigger ones will gain the benefit of using their Growth power rank in place of Strength for Strength FEATs, so you don't necessarily have to increase the Strength rank as they scale up. They have multiple eyes, so I would definitely give them vision based enhancement, such as the Enhanced Senses power or, if using the Ultimate Powers Book, the Circular Vision power. They're also squishy and slimy, which could make them more resistant to blunt attacks as well as fire and/or energy attacks... so maybe a low (Poor to Good) rank in Body Armor with the limitation that it doesn't protect against edged or shooting attacks, and also add Resistance to Fire and/or Resistance to Radiation (
which includes energy attacks). They can manifest limbs at will, so I'm not sure how to handle that one... except maybe call it Extra Body Parts that can be activated and deactivated. Then it becomes a matter of deciding what to set their base statistics at.
Your turn: I'm not going to break down everything in the mythos and analyze it like this because I could go over everything Lovecraft and other sources have written on the topic and might be wasting my time if you only need stats for one or two creatures, but this is how to start creating what you need. Look at the source material, figure out how the rules of your chosen game system can apply, and then fill in the blanks as needed for your game. Remember, no version has to be 100% perfect, so even if there were readily available stats for everything, that's just one interpretation of the monsters and it might not be a good fit for your gaming needs. Feel free to adjust as necessary.
And if you have questions about any specific monsters, definitely inquire here to see how others might interpret something for comparison or help you figure out where you should set your own baselines.
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