[Wrana]

Idea seems quite good. Not tested yet, but some thoughts occurred right now:
Using Abyssal in unit(s) name as somebody here suggested. Much more thematic than "pearl".
Generally, I think undersea Blood magic/sacrifice quite thematic. After all, it was present in real world. But - probably the game designers had some reasons for not using it in the basegame, so bugs are quite possible here.
I do not think 4-armed demon is thematic here. However, there are a score of quite demonic-looking or called deepwater animals. E.g., there is a squid called "sea devil" - actually, it's quite small, but... Some extinct animals may also be handy. I'd also look into, say, Polynesian mythology.
Maybe, too, blood vortex should look more like vortex in a sea and less like a tornado?
Maybe more later.