Feeblemind can, on a failed save, reduce a creature's intelligence and charisma to 1. That's a -5 modifier.
Every 30 days, the target can repeat this save. Does the character save with their current intelligence or their original? For a character with a DC spell save of 16, the character might never recover on their own without proficiency in intelligence saving throws. (Thanks to Michał Politowski for pointing out that nat 20's are not automatic successes on saving throws).
I guess the root of my question is that this seems over powered, but I really just want to make sure I'd be rolling the correct modifiers.