What would happen if some one used a destructive spell in mithdanor? Would the realm just fall apart? (and by destructive i mean of godlike proportions) Because mithdranors magic is unstable?
Well the magic/Mythal in Myth Drannor is probably unstable enough...I suppose if a "godlike" destructive event occurred, it's possible that such an event could rupture space time.
If gods are unleashing divine wrath in a realm of ancient unstable elven magics then the results are pretty much whatever the DM wants them to be; no surprise that there aren't rules that cover such a unique event.
If time itself were somehow ruptured then the devastation may only manifest backwards through time instead of forward. It'd be funny to see nothing visible occur in the present day but then learn later that the time-destroying event is what actually destroyed the city long ago in the past.