A Candlestone is an evil crystal that absorbs all light energy and spits out excess light. It works like an anti-prism so it takes individual colors and makes one beam of light. It is also know as a dragons bane because it sucks the light from a dragon's scales.
People can also be trapped in candlestones. Sir Devin was trapped in a candlestone when Excalibur turned his body into light energy. Bonnie goes into the candlestone believing that it is her mother inside but ends up releasing Devin, a group of knights, and some animals that were trapped inside.