Dark Souls 3 Walkthrough: Smoldering Lake, Old Demon King Boss Fight
Once you defeat the High Lord Wolnir in Catacombs of Carthus, you will Lit the Bonfire and then Head back towards the Suspended Bridge and get down to take the Old Demon King and towards the Optional Area - Smoldering Lake. Here you will have to face some new kind of Enemies which includes different types of Crabs, Ghru, Demons and much more. So let's start with the walkthrough.
Abandoned Tomb Bonfire
As you get down from the Suspended Bridge, get inside the cave and you will have to fight against one of the Demon to get past the Area. If you head up after killing the demon you will find a treasure chest which is actually a Minic so you better be ready to take that thing down as you will be getting a Katana or Black Blade as a reward. Head down to fight the room full of Skeletons or you can collect the Orb and head down skipping the fight to Lit the Bonfire.
Demon Ruins Bonfire
Head Down Straight from the Bonfire and you will finally enter the Smoldering Lake area. Now here you will be fighting against Carthus Sandworm which is very fast and hard to deal with so if you want you can skip it for now and head straight and on your Right will be a Batista which will be firing Fire bombs at you. Quickly get passed them and head up from the Right and get inside the cave and Lit the Bonfire before we take down the Demon Boss.
Old Demon King Boss Fight
Now head out and run towards the Smoke and if you want you can Summon Phantom Great Swamp Curculus at the Steps who can assist you with the Boss fight. This King is Weak to Black Knight Greatsword, Black Knight Sword, Black Knight Greataxe, Black Knight Glaive. So make sure you have one if those and use them against him. He will start the fight by Vomiting lava from your left to right, so staying too long Close to him will trigger the fast AoE mouth blast. Attacking only a couple of times and then retreating to medium range to wait for openings is easier.
As you drain his health to 50% he will be changing his attack pattern, but you can concentrate on the same openings as in the first phase. Keep moving towards him until he does a melee attack and whenever you see him lowering his club for the slow swipe then start attacking just like you did before. If it is any other melee attack you can dodge or even walk under him to get behind him and hit his Tail. Once he is down, Lit the Bonfire to Save your Progress and Updates.
Old King's Antechamber Bonfire
You can Fast Travel to Smoldering Lake Bonfire(Bonfire just before the Boss) to head towards the next area. Head to from there and you will come across some new types of enemies called Ghru with Fire Orb on hand so you need to kill then fast before they raise the Orb in the air. Head right and continue towards the left and you will find some more Ghrus but these are different and they poison you so be careful.
There will be two options at the end of the Room where you can either head downstairs to the whole new area with Enemies surrounded or take a left and jump to get the Bonfire. If you take left then remember to block some lava Rotten Flesh then you must get ready to make a good jump, but if you fell down then you will enter the area which we avoided previously(Downstairs). Now there is another way to get here from the Demon Ruins but these is what we find the easiest way to grab one.
Now it's time to head down to the Room filled with enemies and explore the area as we have already taken down the Boss. The Area is more dangerous than the actual boss so stay alert. You will be hunted by Some Ghru, Fire status and finally Black Knight inside the Illusory Wall on the Left. Now we would recommend you to use the Fire shield as the next room will be full of Fire enemies. Head the first left and drop down to kill the Hound-Rats.
Now if you are Running low on health then climb up(left) and jump the Lava Pit and head towards the Demon Ruins Bonfire or Hit the Right Illusory Wall where you Dropdown, grab the chest and again hit the Wall ahead to explore the new area. There is now need to kill anyone here as they do not drop any items and before climbing the stairs we recommend you to take left and head down to fight Knight Slayer Tsorig and grab the Black Iron Greatshield.
Head Upstairs and get ready to take down another Dark Knight then climb the Ladder which will take us to the actual part of the Smoldering Lake. As you reach upstairs head left so that you avoid getting hit by the Batista. Take down all the enemies on your way up, we recommend to take them one by one for a smooth kill, then Disable the Ballistic.
Horace the Hushed
Now head down and then back left where we will be fighting Horace. Take him down with ease and grab the Llewellyn Shield and the other orbs. Now we are done with this Area only a few loots which you need to pick up from the Lake if you manage to Survive against the Sandworm. To Continue to the next area head to the old King Demon's Bonfire and continue the Irithyll of the Boreal Valley Walkthrough. Don't Forget to visit our Dark Souls 3 Wiki page to know more about the game and its collectibles.