There is a doorway here you can take inside. There will be several slugs and Pus of Man beasts in this mist swamp so be cautious when exploring. Defeat it and explore the surrounding toxic area for a Claw, the Shadow Set, Black Firebomb 2x, Human Pine Resin 1x, Titanite Chunk 1x (up staircase), Ring of Sacrifice, Dark Gem 1x. Like others encountered prior, it is weak to fire. The staircase leads down to a toxic mist area where a Pus of Man beast patrols. If you head left, you can drop down to a narrow staircase with a Titanite Chunk 1x.

Exit the lift and head right down the stairs to find an Estus Shard 1x. Defeat it and enter the room to its right and take the lift down & roll out to a landing leading 1/2 way down. Head down the staircase (grab an Item to the right) to come to a Cathedral Knight, similar to the ones in Cathedral of the Deep. If you head left before entering Lothric Castle proper you will reach the area. This area can be accessed after defeating Dancer of the Boreal Valley and unlocking Lothric Castle via the ladder.

Crusader kings lord of the rings