Re: Nanobot Cloud
Since the "cloud" dismantles ordinary metal into atoms, then reassembles them as more nanobots, perhaps their ultimate weapon is just making enough nanobots so the next stage in their programming may commence.
Stage One: Consume all metals on the planet, turning them into nanobots.
Stage Two: the Nanobots, once sufficiently numerous, alter the balance of gases/liquids in the atmosphere/ocean to what the badguys prefer to breathe/drink.
Stage Three: Convert all organic matter (including unprotected people) into organic "nanolegos" (whatever the building blocks of life are on Planet Badguy).
Stage Four: Nanobots assemble these organic nanolegos into a duplicate of the Badguys' biosphere, right down to the plants, animals, fish and fungi.
Stage Five: Upon completion of Stage Four the nanobots build factorys/housing/office space for the new badguy overlords.
Stage Six: When complete, the nanobots clump together and deactivate, leaving behind easily exploitable piles of raw material.