The Chamber of Candles was originally a dark dungeon made by an evil squirrel king and his followers. He would throw his enemies into a deep pit dead or alive. Some have been known to escape. Husk later found the sealed tunnels and kept them secret. The darkness and evil in the tunnels fueled his evil desires. He would cull the young there and throw them into the pit. During the battle against Husk, he escapes Crispin through the tunnels. When he sees a light at the end of the tunnel, he panics and notices the ghost-like figures of Urchin and Hope (Whom he assumes to be Prince Tumble) and stumbles back into the pit. His skeleton is found by Juniper, Urchin, and Needle later in the series [The Heir to Mistmantle] when they venture down into the pit and find an escape route dug by animals who had escaped. After Husk's death, Brother Fir transformed the tunnels into the Chamber of Candles by filling them with light and love. They are mostly used for worship and prayer, but during the Raven War they are used as a place of refuge.
Before it became the chamber of candles, these tunnels were completely dark, cold and seemed to have oozing slimy things spread across the walls and floor. It had a strange pull of evil and darkness to it. When Urchin first followed Husk there he felt/herd unseen things running past.
After Brother Fir cleansed it, the chamber of candles warm a dry. It's candles give a comforting glow. In Raven war Brother Fir was moved there to keep him safe.