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!
Containing 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.
Ah, very good. I'll need these to contain the fires of the candle. Now that I have the scales, I'll make the candle and hold onto it until you're ready to begin your ritual for the Circle of Greater Summoning.