Sudarshana Chakra is a weapon used by Lord Vishnu. It is a spinning, disc-like weapon. The chakra comprises thousand spikes in two rows (sahasraram). One row of spikes moving in the opposite direction give it a saw-toothed edge.
The Sudarshana Chakra is the only divine weapon which is constantly in motion. It is believed that Sudarshana chakra can travel as fast as the thoughts.
Sri Sudarshana Moorthi is being worshipped while doing Sudarshana Havan and each ahuti is offered with a prayer to remove negative aspects within and around you. The havan kund is considered as His mouth and the fire, his tongue.
Usually a herb called 'Kadalaadi' (Achyranthes aspera) is offered along with ghee (10% of the total number of the herbs offered) into the fire, chanting appropriate Sudarshan mantras (there are various Sudarshana mantras for different needs).