Halo

Halos are rings that often are used on angels. Halos hover (or are kept up by some object) obove the head of the angel. On board the Titanic when the angelic robot workforce known as the Heavenly Host turned against the crew they used their halos as weapons. They were programmed to scour the ship and use their Halos as deadly frisbees to assasinate any survivors of the meteoroid storm. The Halos could travel at incredible speeds, enough to decapitate a human. The Doctor and his fellow surviving passengers used chunks of stray metal to rebound the Halos and as it turned out an Electromagnetic pulse could not only disable the Host but the Halos as well. (DW: Voyage of the Damned)