The Smouldering Lake is one of the optional areas you can access in Dark Souls III. In order to get to it, you must have first defeated the Abyss Watchers, Lords of Cinder. This will give you access to the Catacombs of Carthus. After crossing the shaky rope bridge that takes you to the High Lord Wolnir boss fight, turn around and swing your weapon at the rope bridge. This will cause the bridge to collapse, creating a ladder that takes you deeper into the Catacombs.
Climb down the ladder, and you will come into a room with a large fire demon. There is also a chest sitting on the top platform, but be careful – it’s actually a mimic. You can trigger the mimic, lure it downstairs, and the fire demon will help to kill the mimic for you. Skeletons will also trigger from below, and they can help you fight the fire demon as well. If you don’t want to engage any of the enemies, simply run past the skeletons and you’ll soon come into a cave-like area with a bonfire. Light the bonfire, proceed onwards, and you’ll reach a lava-filled area that looks a lot like Izalith from the first game.
The Smouldering Lake is home to an optional boss, as well as some valuable lore items that will answer quite a few questions from the first Dark Souls.
Be sure to check out our Dark Souls III wiki for more tips and tricks.