I don't know if this is a spoiler or not, but I'll just say it is to be safe.
In the novel the author made a note explaining something about the merfolk:
So, because of global warming the females(Mermaids) couldn't successfully give birth and would get miscarriages. So, as that went on and more and more females had miscarriages the males(Merman) decided that females were useless. The males then went onto eating the mermaids instead of mating with them. And how the merman would "reproduce" was basically going up to the surface and finding a male human and having s*x with them because merman c*m could turn a humam into a mermaid. 💀
Anyway that's why Agares ate Lilith and is trying to mate with Desharow!
