So I succeeded the DC99 check with a crit success (it was my only success in these checks). Later, at the end, in the very final fight, the netherbrain started with about 60 less hp and a debuff that said that it was very surprised I could succeed this check (not the exact words but you get the idea).
Iirc it started with 246hp instead of 300. I don’t remember if the debuff did something else.