Unbreakable Oath

You present a contract to up to six creatures with whom you can communicate. The terms of the contract must be, but those signing do not have to know they are entering a magical contract. Any attempt to force or coerce a creature through magical terms breaks the spell. 

Once signed the creatures must complete the task within the time frame given. If a creature attempts to break the oath or go against it of they do not complete it in the time allotted they must make a Wisdom Save. On a failed save the creature takes 2d6 psychic damage and one point of exhaustion. This continues every dawn until the task is complete, the spell is removed either by the caster, dispel good and evil, greater restoration, or a wish spell. 

On a successful save the creature is free from the oath,  but immediately takes 1d6 psychic damage and an additional 1d6 every minute it is within 60ft of the caster for 1 year

At Higher Levels

The number of people affected increases by 1 and the damage increases by 1d6 for every spell above 5th level. You can also choose to turn the creature undead if they die in the course of completing the task.