The Smouldering Lake is a shallow lake that lies deep underground. Enormous trees that smoulder with embers grow up to the ceiling and their roots reach far into the chambers below the lake. It has been known to be the remains of the once molten Lost Izalith, or an area adjacent to the lost city. After many years, the lava had subsided significantly; the evidence to this being the presence of the demon corpses that litter the ground in the Demon Ruins, which in Dark Souls was the area one would have to go though to reach Lost Izalith.
This was once the home to the daughters of the Witch of Izalith and the demons born from the Flame of Chaos. Unlike the First Flame, which is tied to the souls of the undead, the faded Flame of Chaos was tied to the souls of the demons, and so the lifeless bodies of Capra, Taurus, Asylum and a single Centipede Demon can be found strewn throughout the ruins and piled high near the Old Demon King, one of the last remaining demons of old. A petrified arachnoid body, resembling Quelaag's Sister, lies within the ruins, where a humanoid corpse holds the Quelana Pyromancy Tome, indicated that the sisters lived here at one point.
On the highest ledge above the Smouldering Lake, a colossal triple ballista fires giant bolts at anyone trying to cross the lake. It is fully automated and the Giant Slaves that maintained it are long dead. It represents a significant threat; however, it can be used to one's advantage by getting a bolt to hit the unstable floor in the middle of the lake, opening an entrance to the Old King's Antechamber. Additionally, at the far wall left of the ballista is a crumbling wall which hides the Speckled Stoneplate Ring, and demolishing the wall with a bolt allows access to it.
Another entrance to the Demon Ruins lies straight ahead from the entrance to the lake, near the chamber of the Old Demon King. However, burrowed into the ground nearby is a Carthus Sand Worm, which will shoot up when someone comes nearby. As bolts from the ballista can also reach this area, it is an extremely dangerous route to take.
Inside the Demon Ruins reside Smouldering Ghrus, Demon Clerics, Demon Statues, Hound Rats and Giant Hound Rats, Basilisks, and two Black Knights. To reach the upper reaches of the lake, a long, winding route through the ruins must be taken, with a great number of these enemies along the way.
In the deepest part of the ruins, above a pool of deadly lava, there is a ladder leading up to the high ledges above the lake, guarded by a Black Knight. Below, Knight Slayer Tsorig can be found. Once emerging on the ledge, a few ballista bolts must be avoided, but the rock formations will prevent most of them from reaching below. A few Skeletons, Skeleton Wheels and Skeleton Swordsmen patrol this area. Past them, the ballista is spinning its gears and can be deactivated with a lever, making the lower area of the lake substantially safer to traverse. A series of ledges against the wall can be dropped down to reach the bottom quickly.
An unconfirmed, yet possible, theory is that the Smouldering Lake is another section of the Ash Lake from Dark Souls. This theory is supported by the presence of massive trees, resembling the archtrees that the ancient dragons once lived in and that the witches of Izalith burned with their pyromancy. It is also supported by the close proximity the Ash Lake had to the Demon Ruins as well. Looking from a top-down view from Blighttown, one could see that the entrance to the Demon Ruins was right across from the entrance to the Ash Lake; and as both were beneath, then it would seem reasonable that the two areas were in close proximity to each other.
- Main article: Catacombs of Carthus (bonfires)
- Black Knight
- Demon Cleric
- Demon Statue
- Giant Hound Rat
- Great Crab
- Hound Rat
- Lesser Crab
- Skeleton Swordsman
- Skeleton Wheel
- Smouldering Ghru
- Writhing Rotten Flesh