The Circle of Fire is a special ring in Dragon Quest V.
Finding the Circle of Fire is one of the two tasks Rodrigo Briscoletti requires before asking his daughter's hand in marriage, and can be found in the depths of Mount Magmageddon.
The ring is guarded by three magmen. It becomes the Hero's wedding ring after getting married, and can be used as a tool in battle to cast Boom on opponents.
Later, it is revealed that this ring (along with the Circle of Water and the Circle of Life) is one of three required to unlock the portal in the Estuary Sanctuary that leads to to Nadiria.