I am dming a game and wanted to let my players find a powerful spell with a high cost for them to debate over using. I would love some feedback on it before I accidentally give them something worthless/way to powerful.

The spell is listed in the homebrew section as "TichTochs Ritual of The Mages Soul" but the general gist is (level: 5th Target: self Duration: until dispelled Casting time: 24 hours Components: V,S,M A glass knife and a vial of hags blood)

The spell lets players choose a sorcerer spell and add it to their spell list at a permanent cost to their constitution

at 5th level a 1st level spell at 1d4 CON damage

at 7th level upto a 3rd level spell at 2d4 CON damage

at 9th level upto a 5th level spell at 3d4 CON damage

I was worried the cost was to high for what players get out of it but I was also worried players could become super over powered from it.

A thanks in advance for any feedback... also if you like it feel free to use it.

I don't think they need to take CON damage from this spell. It doesn't let them cast the spell without a slot or anything.

What it does need is a restrictive material cost. Something like a 1000 gp worth of potion ingredients and a hag's heart which the spell consumes.

I see. I really want the spell to have a high cost so that players have to really consider if they want to use it (in the realm of a permanent stat debuff). But you're saying the reward is too low?

So your advice would either be:

1) Allow the selected spell to be cast as a cantrip

Or

2) keep the current spell reward but limit the cost to things that are rare but not irreplaceable