Devil's Bargain

 You and a willing creature that can understand you enter into a supernatural agreement. A physical mark of the deal manifests on each of you until the spell ends. A creature that breaks the agreement or deal is cursed. Roll on the long term madness table to determine what form the curse takes. This effect lasts for the duration of the spell. 
The target need not know about the consequences of breaking the deal in order to agree.  
A greater restoration spell, wish spell,  or remove curse spell of the same or greater level as the original casting ends the spell.