That's a good point, I didn't really think about that. I don't really need to destroy the village, the PCs could even just encounter the adventurers mopping up parts of the horde outside the intact village.
Some further information.
I wasn't really detailed enough in my explanation. They are going into the monastery to assassinate the leader of the horde, and to attempt to wipe out the horde itself, as no one knows its true size and the towns folk suspect it is a group of a couple dozen since so far the horde has just been making surprise attacks in the night. The PCs will also find a mysterious artefact in the monastery of odvious value but unkown purpose. I was actually planning on having one of the NPCs being in the employ of the lord and recognizing the artefact, I just forgot to put it in the post. What I could do with the binding the demon lord part of the plot is have the ritual simply summon the Demon Lord, perhaps the Lord Wilhelm could be arrogant enough to think he could control the demon when in actuality he couldn't.
With the kind of characters that my friends play, actually going through with the ritual itself is a likely option.