Devilish Deal

You and a willing creature, who cannot be under the charmed condition, enter a binding pact. The caster and the other party are bound by a ancient force to complete the terms of the contract. If a party is unable to complete their end of the contract, they take 10d10 fire damage that cannot be negated or resisted in any way, and the contract becomes null and void. If a soul is bartered on one end of the contract, that party's soul with leave their body at the specified time and enters the diamond. The only way for a soul to be released from the diamond is for the contract to be signed over to someone/something else (relinquishing the soul to the party to which it was signed over). If the soul is relinquished to the original owner, it enters their body if alive, or their destined afterlife if they are deceased.