Curse of Tongues

You steal the target's voice with a curse, replacing it with the voice of a terrible demon. The target must make a Charisma saving throw or become cursed. While cursed in this way, any attempts to communicate result in them speaking in the eredun language, whether or not they understand it. If they do not speak eredun, they must make check with their spellcasting ability modifier with DC equal to 15 + the spell's level whenever they try to cast a spell with a verbal component. On a failed check, the spell is lost.

Regardless, casting beneficial magic in the language of demons is taxing. Casting a spell that deals holy or radiant damage, removes a condition, buffs an ally, or restores hit points to other creatures costs twice as much mana, unless you give yourself disadvantage on attack rolls and your targets advantage on saving throws while casting it. You may not make this choice if you already have disadvantage on attack rolls or if any of your targets have advantage on the saving throws against your spell, or if the spell does not require attack rolls or saving throws.

The GM is the final arbiter of spells that are affected by this spell.

Dispel magic has no effect on this spell, but remove curse or other similar abilities can remove it.

At Higher Levels. If you cast this spell using a spell slot of 5th level or higher, the duration is concentration, up to 10 minutes. If you use a spell slot of 6th level or higher, the duration is 8 hours. If you use a spell slot of 7th level or higher, the duration is 24 hours. If you use a 9th level spell slot, the spell lasts until it is dispelled. Using a spell slot of 5th level or higher grants a duration that doesn't require concentration