After the signing of this contract, you are heretoforth to be bound to my command, and stationed here to guard this location, and obey all of my given orders, and no others, until such a time as i see fit to release you from your bound duties. Under no circumstances are you to leave the designated area, defined here as the physical borders of this church, comprising an area of X square feet, according to the blueprint annexed on document 1.B. Failure to comply with any of the terms of this contract will result in immediate Annihilation, with no possibility of reversal, even by such a means as a Wish.
You are to remain bound even in the event of the death of this contractant, being required to be perform your duties adequately according to the last orders given, and the general clause of guarding the church, as defined on the document 1.B, against external threats.
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The flaw in this specific contract is that it defines the area that he needs to protect according to the blueprint annexed on document 1.B, and that he is bound to the area defined by the physical barriers of the church, meaning that if in any way some of the walls of the church were removed, or if the church were to be extensively reformed, it would cease to comply with document 1.B and therefore the contract would become immediately void, releasing the demon from his services.
A case could be made either way, and this is an interpretation loophole that can be questioned later, for another adventure hook.