She prayed on a seventeen year old, a child.
She isolated him from his family and community.
She manipulated him into taking a bad deal because she knew he'd be too stressed at suddenly having the fate of his city (and maybe the world) on his shoulders to argue over his contract.
Never allows him to keep a copy of said contract which he couldn't possibly memorize. It seems like every time she shows up she pulls another clause out of her arse so this thing sounds long and complicated. Plus, legal documents are famously complicated and dry and would not be easy to remember.
She further isolates him by not allowing him to explain himself and their relationship to others but popping up whenever she wants. So he HAS to be the kindest most goody toe shoes person possible otherwise people will just reject him or worst when she shows up. Like it's great for Wyll that he's incredible at deception but is not a deceptive person otherwise you mightn't trust him when she shows up but there's truly no question that he's not a good person.
She replaced his eye with a sending stone. He literally can't escape her. That would make me feel so paranoid, incapable of feeling truly alone. It's so violating. She replaced part of his body with something of hers. He literally has a part of his abuser imbedded into him. It kind of reminds me of Astarion's scars.
She treats him like a slave. He is her slave essentially. He has to work for her with no real way out. And she hammers home his powerlessness by comparing him to an animal and her pet.
She tries to seduce his friends, driving a wedge between them and making him more isolated.
She owns his soul. He will be her property for all eternity in the Hells in whichever form she chooses, probably a lemur and definitely still a slave. He probably felt a little relived to be illithud because if he turned he wouldn't have a soul she could take.
She tricks him into saving her, the person he most wants dead.
Ransoms Wyll's only family for his soul.
She WILL NOT LEAVE your camp. Can you imagine if Cazador or Shar just hung around Astarion or Shadowheart's tents for WEEKS. It would be insane. They would be traumatized. But Wyll doesn't even get to react to it because of his lack of content due to the rewrites!
The cherry on the cake. The thing that got me thinking about all this in the first place. I'm playing an evil route atm. So I killed Karlach hoping to turn Wyll to a darker path like they promised in this article https://gamerant.com/baldurs-gate-3-wyll-early-access-story-change-karlach-explained/. But Larian would rather fix a million tiny issues than their black main character's story. But the point is Wyll killed Karlach and Mizora comes to reward him for a job well done. And she rewards him with THIS:
The infernal robe. A rare piece of CLOTHING not ARMOUR CLOTHING. Default Wyll can wear light armour so this isn't that useful for him but do you know who works really well if she wears clothing instead of armour? Karlach. Mizora rewards Wyll for killing Karlach with something useless for him but would of been great for her.