The Final Summoning

You utter blasphemous words of power to call upon the darkest creatures in existence. Make a DC18 Intelligence save. On a fail, your soul is trapped in the Far Realms for eternity to serve as a slave. On a success, you successfully summon a random Elder Evil. This Elder Evil has it's power stored in the 7 gems. If they are all destroyed, the Elder Evil is banished back to the Far Realms. This Elder Evil will likely give you a great boon or help you in some way. 

Due to the evils involved in summoning an Elder Evil, you will likely need many warding spells to prevent gods and their servants from interfering.