A demon, of course. Not a particularly powerful one or it wouldn't have used such an intermediary to begin with. Gifting power in exchange for a soul isn't too difficult, but revoking power and claiming a soul can be a bit tricky even with magic on the demon's side of the Deal.
On the occasions someone has bartered their soul for power from me, I bound the power directly to their soul because I have the strength and knowledge to easily claim their soul as my final payment if they violate their terms of the Deal, revoking the power at the same time. Binding both to such a fragile item is an insurance policy for a demon lacking both or either since it'd be simple to shatter the vessel and claim both the power and their contractee's soul.
You breaking the reliquary effectively ended the Deal along with breaking the botched spell. Therefore, your mad monk is either serving their demon for all eternity or having his immortal soul drained of every scrap of power the demon can rip from it. And the former would be preferrable if the demon is as weak as I believe them to be. It will not be a fast or painless process, a Hell all its own.
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On the occasions someone has bartered their soul for power from me, I bound the power directly to their soul because I have the strength and knowledge to easily claim their soul as my final payment if they violate their terms of the Deal, revoking the power at the same time. Binding both to such a fragile item is an insurance policy for a demon lacking both or either since it'd be simple to shatter the vessel and claim both the power and their contractee's soul.
You breaking the reliquary effectively ended the Deal along with breaking the botched spell. Therefore, your mad monk is either serving their demon for all eternity or having his immortal soul drained of every scrap of power the demon can rip from it. And the former would be preferrable if the demon is as weak as I believe them to be. It will not be a fast or painless process, a Hell all its own.