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.

As a Weapon
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. They also seemed to have a remote link to their users, as they would relentlessly bounce back towards their target if they were deflected. The Doctor and his fellow surviving passengers used chunks of stray metal to rebound the Halos, but only an Electromagnetic pulse could disable the Halos along with the Hosts. (DW: Voyage of the Damned)