Need help balancing a homebrew rule for magic

Ok so for context, in my setting there's people called "the sidhe" who are very much inspired by the sith and dark side of the force from star wars. Their whole thing is they're magic users who give in to their emotions and hatred while casting a spell, this causes the spells to be far more powerful but it comes at a cost of altering your physical form and you can sometimes gain illnesses based on whatever magic you cast.

For example their is a warlocks patron who is a Sidhe, she started off as a regular human but was manipulated into using this type of magic, her magic was flavored to be very bug themed and so her illness was throwing up bugs and such before her physical form altered into her becoming just a mass of bugs, unrecognizable from even being human.

In my current campaign my players know they will not be becoming a Sidhe themselves or at least I won't let them as I don't have rules that work for players, only NPCs. But I think this concept could be fun to play as a player I just can't figure out how to balance it.

So obviously as I said the magic is more powerful, so say for example fire ball which ordinarily does 3d6 at lvl3 may now do 6d6. But this insane power has to have a cost, the cost I mentioned above is mostly cosmetic so I'd need there to be a mechanical cost. Perhaps making their spell slots permanently halves? So they can at max only get 2 slots on each level. That could just end up not being fun for the players though is my main worry.

I'd also have it so the players know exactly what they're getting into as I can see this going wrong if the players don't know and it must 100% of the time be the players choice to become a Sidhe.

But as I said I'm struggling to find ways to balance this, so do y'all have any tips?