Deal with a Devil (Mephistopheles)

For better or for worse, you have decided to contract a deal with an Arch Devil. Whether directly or through a subservient demon, you now have gained great power... at a hefty price. This price is determined by the DM

Mephistopheles has immense knowledge of the Arcane. With his assistance, even the most mundane of creatures can cast spells that even power mages struggle with. Choose one level 9 spell from the Wizard spell list. Once per long rest you may cast that spell without using a spell slot. Intelligence your spellcasting ability modifier for this spell.