Dagon

Then suddenly I saw it.

With only a slight churning to mark its rise to the surface, the thing slid into view above the dark waters.

Vast, Polyphemus-like, and loathsome, it darted like a stupendous monster of nightmares to the monolith, about which it flung its gigantic scaly arms, the while it bowed its hideous head and gave vent to certain measured sounds.

I think I went mad then.