Conjure The End

Upon casting the spell, beware, you will summon a creature beyond your control. This creature cannot be controlled once conjured, but it may be dispelled. You may dispell it at anytime, but if it is not dispelled after 15 minutes of the end of the ritual, it will lose the affect you have on its consciousness, and it will be immune to whatever little control you had on it. It will do then as it pleases until it is defeated.

Pick a creature from these:

Tarrasque

Empyrean

A minor god of your DM's choosing