Harald uses conqueror to resist the transformation -> sets up a member of the SH being turned by Imu and resisting the transformation ?
Simpler answer: Harald was stupid enough to get the Mark of the Beast.
Harald got a Mark of the Abyss, and, at that point, Elbaph was kinda doomed.
This is good writing: Harald betrayed Rocks by siding with an organization run by evil human sex traffickers.
Harald chose the greatest evil in the world, over his Pal Rocks, when Rocks would never go after Harald.
Harald was a true fool for believing in the WG, and Elbaph suffered for his stupidity.
I'm pretty positive Harald turned into a Demon through Domi Reversi via having the Mark of the Abyss: he got Gunko'd.
If Harald didn't have the Mark, his CoC resists Domi Reversi.
But: Harald made a deal with the devil, and the Devil came to collect.