There's a short story called "Even Hands" about Marcone's encounter with one of the actual Fomor. They are nothing to sneeze at.
I'm drawing a bit from Lovecraft to draw them up for my game. The "Innsmouth look" is basically, what the servitors are all about, Humans half turned into freakish fish monsters.
The actual Fomor I use are part
mythology, part just the general idea of entities so nasty, they were banished by combined forces of humans and supernatural alike. They've come up every now and again through the ages, and every time they were pushed back at great costs on all sides. And now they're back. With a vengeance.