In the lore I have created for my own personal setting, some of the spells are considered "dark magic", and using them can have some nasty side effects on the user, so they must do a wisdom savings roll to avoid their personality darkening, or mutations that will turn them into a monster. But I've also added some spells into a form of magic called "high magic", which can undo the effects of Dark Magic. What kind of roll do you roll to undo harmful dark magic effects? Fraid these effects are too powerful for Remove Curse. I want players to roll a dice if they wish to undo dark magic side effects. What kind of roll does this warrant?

Possibly Wisdom checks? See Ceremony.

Thanks.

You could flavor it however you wish, using Wisdom (the world-wise understanding of how to undo such effects) Intelligence (a more science based understanding of how Dark Magic infects and thus the countermagic for it) or even Religion, to indicate a Divine intervention type removal of the effect.  I would suggest basing it on something the players have a solid base in, so if there's a Wizard as primary caster, Int based, but if Cleric or perhaps Druid, Wisdom based.

I see.

If you're casting a spell to achieve an effect, any effect, and this casting involves a roll then that roll uses your spellcasting ability. That's how it works, as a general rule. Not saying there can't/shouldn't be exceptions, but unless you have an overriding reason I'd just stick with the standard way of doing things. If you do want to tie this to a specific ability rather than to a class' spellcasting ability, I'd probably flavour it as an effect separate from the casting of the spell - maybe the high magic creates a connection between the subject and a healer, after which the healer is tested in some way. That test could take any number of forms though - could be a test of being attuned and centered (Wisdom), of imposing your will on the subject (Charisma), or simply one of endurance (Constitution), for instance.