Infernal contract in Crown of Fangs

[johstas]

Greetings everyone,
As our party just concluded the adventure path, we are considering our options regarding the fate of Crown of Fangs. One of the PCs (who is a half-fiend) suggested that we let him take the crown to hell and promises he will never return to Prime Material Plane and not ever give away the item, both to the best of his ability. The rest of the party plans to try and find a "holy sword forged by a once mortal God" and then summon the half-devil to destroy the artifact.
Is there a way we can make sure he will keep his promise? We were thinking of an infernal contract but is it even possible for a devil to sign an infernal contract like that? And can we write the contract rather than the devil (I mean the wording and everything)? And what if he breaks the contract? What are the consequences?

Thanks for any assistance

[Knick]

Well, Serithtial was forged by Iomedae, and it is pretty holy, so this may be a moot point. What to do with the Fangs of Kazavon, however...

[johstas]

I don't know how our Dm missed that. Thx a lot for pointing it out Knick.

[Nargy]

Consequences should be outlined in the wording of the contract.