Read it well without skipping
Well basically Desharow is Agaras's son by technicality but since mermaids don't follow the same socials conventions as humans he is his mate as well. This was done On purpose by Agaras's with Desharow being his mate later in mind when he is grown up NOT when he is a child. There is no grooming Agaras's let's Desharow grow into an adult normally before making the first official contact when he gets captured on purpose. He was watching Desharow grow up from a distance and met him a few times when he was a kid and his grandpa was alive but Desharow does not remember till later.
How the descendant thing works is since the mermaids can't reproduce naturally anymore and their nest where new mermaid babies are born does not have enough power to make new babies they want to transform humans into mermaids permanently or let their genes be carried in other humans so when they procreate the child technically belongs to the mermaid. And yes the transformed human also counts as the descendant of the. Mermaid who changed them.
As a novel reader I have access to all kinds of info (〜￣△￣)〜
Descendent and mate??? Did I miss something big, like I mean I got too impatient and skipped a few(80)chs, but like what's happening, is it like a translation error