The Deep Ones have established relationships with humans, strongly desiring to mate with them and breed hybrid offspring. This seems connected to their reproductive cycle. When Deep Ones are worshiped by humans, they regularly mate with these humans; as long as they are not killed, they and their hybrid offspring are immune to disease.
These hybrid offspring usually live in remote seaside villages. During childhood, they grow like ordinary humans but become increasingly ugly as they age.
Finally comes the transformation stage, an extremely painful process for the offspring, as they morph from human appearance into a Deep One resembling a large fish.
Humans are like this, let alone the sea-dwelling Naga sirens.
Naga are easily subjugated.
Of course, not by humans, but by some extremely powerful and terrifying creatures, like the Deep Ones beneath the lakes. As long as they are willing, by killing a few sea sirens, Naga become their servants.
