The Demon Ruins were, before the Chaos scarred the city, most probably, the outskirts of Izalith, home to the famous Witch of Izalith. When the Chaos imploded in the kiln of the city, this area was severely damaged by it, leaving only ruins and mutating the population into chaos demons.
Accessible from Quelaag's domains, which appears to have been a church. Further ahead, the place is terribly deformed, with writhing bodies and terrifying harbingers of death scattering the landscape. An enormous lava pool, emanating from Ceaseless Discharge hides below it a grand army of demons. The mutated, giant Ceaseless Discharge, son of the Witch of Izalith mourns the death of one of her sisters here.
Down the ruins, Kirk, Knight of Thorns invades any trespassers in the heat of the molten rock. Burrowing Rockworms, Taurus and Capra demons guard the remaining edification, where within the Firesage Demon can be found. In the deepest part of the ruins the Centipede Demon rests in a large lava pool.
With the slaying of the Centipede Demon, the Orange Charred Ring can be acquired that once belonged to Ceaseless Discharge, gifted by her sisters, to ease his agony from his fiery sores, allowing to traverse the lava.
The gates of Lost Izalith lie closed but there is another route to the city after the defeat of the Centipede Demon.