Legend

You have no way of getting rid of the imprisoned demon, the only thing here that can is the Black Book, and not being an evil priest you don't understand it. The only thing you can do is free it and then kill it, which is a risky proposition.
Not destroying it will mean part of the forest will still be corrupted, but it won't spread anymore since the Priestess is no longer around to direct the demon's power, of course there's also the possibility of it getting free

Then again perhaps you really don't give a damn about all this and you were just hoping to find some treasure in here only to be severely disappointed.