When first reaching the area, collect the Large Titanite Shards strewn about while avoiding the ballista shots and Great Crabs as best you can. If you head along the right wall from the entrance. you'll reach a small cave entrance. Inside wait two Crystal Lizards and Horace the Hushed, who will attack you.
Kill him to advance his questline and return to the main cavern. Next it is recommended to cross the room to the brick wall. Stand in front of it to beckon a ballista shot which will break the wall, allowing you to collect the Speckled Stoneplate Ring behind. Head to your left from there, you'll see the Carthus Sandworm burrowing through the rock. The easiest way to kill it is to head past it and skirt the wall on the left side, then get on the other side when there's an opening. On this side, take shelter behind the rock and watch the ballista slowly kill it. You'll receive an Undead Bone Shard and the miracle Lightning Stake.
Behind you you'll see the boss door for the Old Demon King; ignore that and head to the right, where you'll find the Demon Ruins bonfire.
From here you can either face the boss first, or clear the area. For the boss, there are two NPC summons available, Knight Slayer Tsorig (if you defeated his invasion in the Catacombs of Carthus and Great Swamp Cuculus, a pyromancer. It is recommended to summon the pyromancer, as if he survives when you beat the boss, you will be able to find Cornyx's set and the Spotted Whip in Cornyx's original cage in the Undead Settlement.
For the boss, bring your best fire-resistant gear and the Flash Sweat pyromancy. Most of his basic swing attacks can easily be avoided by dodging towards him, but be wary of his powerup attacks in the second stage which release huge AOE's all around him.
The area is one of the more difficult ones for its point in the game. Again bring fire-resistant gear. Note there are illusory walls in several locations here:
- In the second bonfire room leading to an Estus Shard
- By a cluster of Hound-rats in the Demon Ruins, leading to Quelana Pyromancy Tome
- In the corner of the hallway that is at the end of the Demon Cleric's room, leading to a Black Knight
- By a cluster of Hound-rats in the Demon Ruins, leading to a chest with another invisible wall just behind it.
You will come across some lava within the Demon Ruins. Within the lava is Toxic Mist, Sacred Flame, a Ember and the White Hair Talisman. To get them, equip the most fire-resistant gear you have, cast Flash sweat, equip the Flame stoneplate ring and Speckled stoneplate ring, bring some of the fire resistance consumable, and have your estus to at least +6. Roll through the lava and estus twice in a row whenever you reach half health. This should help you suicide run to each of the items with enough vigor.
As you progress from the Old King's Antechamber bonfire, you will eventually reach a room full of Basilisks. Past this room, there are two paths: one leads downstairs to Tsorig next to a lava pit, the other leads up to a bridge above the lava pit. In this lava pit are Sacred Flame and an Ember. At the end of the bridge is a Black Knight wielding a Black Knight Greatsword. Behind the Black Knight are two ladders which will take you back to the surface of Smouldering Lake. After climbing up the second ladder, drop back down to a rock protrusion to get the Dragonrider Bow. Then, stay on the path and you will eventually reach the controlling mechanism for the ballista, which allows you to deactivate it. There are two Skeletons, three Skeleton Wheels, and two Carthus Swordsmen wielding shotels on the way. You can simply run past them to get to the ballista if you do not want to fight them. Beyond the ballista, there are spots to drop down and get back to the entrance of Smouldering Lake.