This is epic. Lovecraft stories are definitely a fun thing to illustrate. I really like how you've handled this and it's been interesting to see it progress. In your last image I really like the subtle green on Cthulhu, contrasting with the eerie purple in the rest of the image, well done!
Just some personal nitpicks;
If I were painting this image, I would have turn Cthulhu to be three quarter, or at least shift him a bit so he wasn't dead-on to the camera. It doesn't jive well with the angle of the beach, in my opinion. It looks like you've turned him slightly in the last image, though.
I liked the framing element the fish dude nearest the camera, now that you've pushed him back it's detracting some of the horror aspect in the sense that you can't see so much of his hideous detail and the fact that previously his form made the painting feel much more enclosed and threatening.
Watch out that the fish guys on the left side of the image don't start to look like they're repeating too much. You had a nice sense of variation going on, and in the last image it's looking dangerously copy-pasty.
I also just want to gush about how much I love the sand rendering, as it's something I've never managed to fully pin so far, and I love the long shadows, they add such an air of menace.