Well for one of the main evil races I would probably use Aboleths and have them worship some of the Old Gods and maybe like a couple of new Sea Gods who are all about conquering the land because it rightfully belongs to the dwellers of the sea and all. But I really kind of want like the "native" sea folk like marrow and sea elves and stuff being evil and wishing doom and death upon the dwellers of the land, being evil and all.