I've read the Monster Manual entry for Merrow but I'm trying to get a better sense of them as a species. The book says
Long ago, a tribe of merfolk found an idol of Demogorgon [...] Everyone who touched the idol became afflicted with madness.
[...] the king led the survivors through the underwater gate to Demogorgon's layer of the Abyss. The merfolk remained there for generations [...] transforming them into hulking, evil monstrosities. Thus, the first merrow were born.
So we know how they started as a species. My question is, is there any indication how future merrow are made? Do they just keep pouring out of the Abyss as needed, and/or can current merfolk be somehow turned or corrupted into new merrow?
I could obviously homebrew this to my heart's content but I'm first interested in whether there's supplemental material that expands on merrow.