Lanefan
Victoria Rules
If they decide they want to break the macguffin they're free to do so. Wouldn't be the first time...Unless there was some particular reason to keep them from willy nilly breaking something (like being a macguffin or something), I don't see why I'd stop them from breaking something.
I use a combination of common sense and the 1e save tables to determine how easy something is to break. Snapping a wand, usually trivial. Breaking a heavy weapon, takes effort but usually do-able. Breaking a bridge, bigger production and not always possible. Etc.
That said, breaking magic items in my game risks the item's contained magic going 'boom' or releasing a wild surge, so it's not a recommended tactic unless all hope is already gone.
