A Doomsday Candle stores fire and directs it at those who would disrupt your ritual.  When creating your Circle of Greater Summoning, if your candle is lit then any wayward demons will be burned by its flames!$B$BContaining that fire is not easy, and requires the essence within the scales of a black dragon.  Acquire those scales from a skinner and bring them to Gorzeeki Wildeyes.