This is a rough one, and you want to draw it as early as possible--or have an insurance policy that NEVER leaves your hand. There's been a couple of times where I had almost a full bag of bless tokens, and got hit for 4 horror AND 4 blessings changed to curses right about the time you're gearing up for that boss fight at the end of the scenario. Yeah, I powered through it, but it doesn't feel too good. And for that reason, thematically, I love this weakness.
Revelation - For each token in the chaos bag, you must either replace it with a token or take 1 horror.
This one is going to hurt you, with a few caveats: if your seeker friends help you find it, you can trigger it to do very little horror; if a fellow investigator is trying to use the grimoire, it could turn your tokens right back into blessings; you can use rite of sanctification to keep those blesses out of the bag.
Update: it seems the blessing tokens can be easily sealed. So this weakness is to prevent Mary from just flooding the bag and seeing what happens. As a weakness, it’s pretty interesting how it requires you to work with a rhythm, just like Thrice-Damned Curiosity.
Beat this with deny existence. (Thanks Death by Chocolate.)