Hit the wall opposite the doorway you came out of. Make another right and deal with yet another set of Hound-rats. Make the next right, taking on the Hound-rat along the way. Head down the stairs that’s right of the Demon Cleric, then make a right at the lower area, which happens to feature another Demon Cleric. The first Demon Cleric will be on your left when you emerge from the passage. In other words, the quicker you deal with them, the better. Their frequent use of fireballs pose a challenge because some function as satellite fireballs that shoot their own fireballs. When you emerge from the passage, you’ll face off against a couple Demon Clerics. There's another passage on the other side of this room with the bonfire. This will lead to you to another bonfire, which is a more convenient respawn point for the boss fight. You can choose to take on the boss, Old Demon King, or you can make a right near the doorway and go up the path. You should see another foggy doorway leading to another boss in the distance.
Killing it will give you a Lightning Strike and an Undead Bone Shard. You should either rely on ranged attacks or come back to it later when your levels are high. It’s burrowing ability makes it a tough opponent if you’re using melee attacks. You will also come across a large Carthus Sandworm. Veer to the left of the lake, while being mindful of the massive ballista attacks aiming at you. Grab three Large Titanite Shards around the center of the lake. Head past the bonfire to reach the Smouldering Lake. Head through the doorway at the bottom tier and take the next right to reach the next bonfire. The other side of the top tier includes a Mimic, which will drop a Black Blade when you kill it or when you bait it to run into the Demon’s flames. When you've reached the bottom, head down the path and into the large three-tiered room with the Demon and several skeletons. This will make the bridge collapse, creating a ladder you can descend, leading you to another area of the catacombs. Instead, cut the bridge ropes on your side (one swing will do it). To access a nearby optional area, head back to the suspension bridge from the High Lord Wolnir bonfire, but do not cross the bridge. Click the map to make it larger, and be sure to check out all of our Dark Souls 3 maps.