From what we know, mohg snatched miquella from his cocoon in the haligtree while he was trying to break free of that eternal youth curse. The interesting part is, we all thought mohg was just being obsessive trying to make miquella into a god and become his consort but the whole story's gotten way more complicated since the dlc came out.

Turns out miquella was actually pulling the strings the whole time, using his charm powers to basically trick mohg into taking him. What's really fascinating is how miquella had this whole plan worked out - he needed to get to the land of shadow, and mohg was basically just a stepping stone to get there. The poor guy thought he was making this grand dynasty in his bloody bedchamber, but he was just being used.

Here's where it gets even darker - miquella wasn't just using mohg as a ride to somewhere else, he was planning to use his actual corpse as a vessel for radahn's soul. Pretty brutal when you think about it, especially since mohg was completely devoted to him. Once miquella got what he needed, he just abandoned mohg which his followers saw as this huge humiliation.

The whole thing really shows how miquella was playing chess while everyone else was playing checkers. He wanted to break away from his family's cursed bloodline, and he didn't mind sacrificing a few demigods to make it happen.