Forget the boss; we have more interesting things to do down here.
Dark Souls 3: Catacombs of Carthus to Abandoned Tomb
The Catacombs of Carthus bonfire is the closest stop on your way to High Lord Wolnir, but there's plenty of treasure to collect and a whole side path to explore before we make a run on the boss.
Make your way down the boulder corridor using the alcoves if necessary, and head right. There are three rats which begin to move from right to left (with your back to the boulder corridor) as you advance down the boulder corridor; once they're settled on the left they don't come backm but be aware that the third rat follows the others at a fair distance.
At the bottom, on your right is a single rat and a giant rat who stay still. Dispatch them before turning your attention to the tunnel at this end. There are slimes on the ceiling, so keep a sharp eye and ear out, but our main distraction is a group of four wheeled skeletons at the far end.
If you didn’t already realise that Dark Souls is ridiculous, welcome to wheeled skeletons. They’re pretty easy to beat once you have them on their own; they just run at you, then climb into their wheels and spin in a straight line. Just step smartly out of the way then run after them and paddle away after they skid to a stop.
As you advance down the corridor, note the item glow. There are multiple slimes above this who will drop if you collect it; it’s a Titanite Shard, by the way. There are two more slimes along the tunnel, and one slime right at the end. Turn the corner and pick out three slimes on the ceiling ahead. After all that, your prize is a Carthus Blood Ring.
Go back to the room where you fought the rats and explore in the other direction. Take the first right to go down some stairs. There’s an Ember here, and a preview of an upcoming area. You can shoot through the grate but it’s really too far to do any good; if you’re lucky the skeletal knight will run over and you can cheese him. If you hang about here in Ember form you’ll almost certainly be invaded by an NPC phantom.
Things get a bit deadlier as we move through the sewer. There’s a shotel skeletal knight ahead. Best to take him down before going for the Titanite Shards scattered around. Remember to smack him with a club to knock him down!
There are three more doorways after the stairs; one is behind an illusory wall. The first two lead upstairs, where all the horrible skeletal things you spotted from the other side of this area are: two shotel skeletal knights, a skeletal mage and a skeletal archer. Up here you’ll find a number of Titanite Shards as well as the Grave Warden's Pyromancy Tome (give to Karla when we unlock her later on) and the Grave Warden’s Ashes (return to the Shrine Handmaid for another inventory expansion).
The last path leads to a Crystal Lizard with a Fire Gem – and then the huge skeletal knight with you saw from behind the grate earlier.
Collect the Yellow Bug Pellets from nearby and take note of the message. What we’re supposed to do is dash through the next room and sprint across the bridge, then chop it down so the enormous horde of skeletons out there don’t kill us. We'll be back here later, so don't feel obliged to grab the Black Bug Pellets and soul as you dash past.
On the other side of the chasm, attack the post on the left (being careful not to fall off) to make the bridge fall. More importantly, perhaps, you can activate the fallen bridge and climb down it like a ladder. There’s a Demon down the bottom; these no longer count as bosses, but you will get a Soul of a Demon for it, which you can trade with Ludleth.
If you decide to cheese the Demon from the door, be conservative; some of its attacks will come through the wall, and its fire breath has a very long range. There are multiple reanimating skeletons and reanimating one-armed skeletons around the boss arena and next level. The treasures are a Titanite Shard and a soul.
If you run around the ledge at the highest level of this area, you’ll find a treasure chest. It is, of course, a Mimic. Kill it for the Black Blade.
Delightful update: As of version 1.03, you can save yourself a lot of hassle with the Demon and the Mimic by making them fight each other. As soon as you enter the room, burn around to the ledge in front of the Mimic and wait for the Demon to do its fire breath attack. This should aggro the Mimic, but if you move away, it won't come for you, and will instead attack the Demon. One of the two combatants will triumph in this battle while taking out huge amounts of the other's health, allowing you to end the battle quickly and decisively.
In the next (safe!) tunnel you’ll find the Old Sage’s Blindfold and the Witch’s Ring. The path to the side will allow you to activate the Abandoned Tomb bonfire.