This Caelid dungeon can be reached after defeating Radahn, and is reminiscent of the Abyss Watchers fight from Dark Souls 3 — enemies here are constantly reborn to fight one another again and again. It’s also got a tough version of the Rot Dragon you’ve seen a few times already for its boss.
This dungeon is pretty easy to miss, since you have to ride to the far northern corner of Starscourge Radahn’s boss arena to reach it.
From the site of grace, continue deeper into the catacombs, heading down the stairs that take you around a dark pit. You’ll then enter a long hallway, where you’ll be able to see some apparitions fighting in the main room to the west.
We’re going to grab some items while attempting to avoid combat in this next section, so if you aren’t fighting or actively running away, you should be sneaking. This area has a bunch of rot, so if you don’t have Flame, Cleanse Me or some Preserving Boluses, you should try your best not to let your rot meter fill. Fortunately, there are plenty of weird white mushrooms that you can stand on while you wait. You can also equip the Immunizing Horn Charm, or other sources of Immunity, to make it easier to not catch scarlet rot.
Enter the room and immediately go left, staying between the wall and the columns. When you hit the wall, turn right to head west to the stairs, but don’t walk down them. Instead, drop off of them heading north into the rot. Turn right and head into the southeast corner to find a Silver-Pickled Fowl Foot — you can also stand on the mushrooms here while your rot meter depletes.
Next, head north into the hallway, but don’t enter it — just stand on the stone and once again let your rot meter deplete. You don’t have to let it get all the way to zero, just wait a bit if it’s near full. Head west from the hallway entrance, hugging the wall as you go around the corner.
Look left as you round the first corner to see a corpse with a Golden Rune . Then continue hugging the northern wall and head all the way west, until you’re in another corner on some mushrooms. Look south — there’s a chest just ahead in the rot (it may be blocked by a column while you’re standing in the corner, but head south a bit and it’ll be easy to spot). The chest has Collapsing Stars, which is an incredibly FP efficient sorcery — if you aren’t interested in magic, you can always ignore it.
An enemy will likely drop down on you while opening the chest, and it’s also pretty tough to not end up with scarlet rot while doing so. Loot it quickly and run away to the northeast and enter the hallway you stood in earlier. In the room to the north, a gargoyle imp waits to throw fire pots at you, and another hides on the wall just to the left of the doorway. Melee characters should rush in and try to kill the standing imp quickly, then turn left to face the other imp (ranged characters can happily kill the visible imp while safely in the hallway).
Once they’re both down, head east behind the statue and kill the imp hiding there, then grab the Ghost Glovewort  across from where the imp was sitting. Now you can head back the way you came, go through the hallway, then sprint south to the opposite side of the room and wait on the mushrooms in the left corner. It’s possible that the spirit soldiers are fighting nearby, so you may have to reroute, but ideally you’ll be able to just go around the corner and head up the nearby stairs (just in case you got turned around: they’re on the south side of the room, in the middle of the wall).
Be very aware of the Cleanrot Knights — if they don’t have any enemies to fight, you’ll want to be extra sneaky while making your way to the stairs. At the top of the stairs, peek around the corner and see if there are any spirits. If none remain, you can grab the x3 Magic Grease off of the corpse in the middle of the room. Otherwise, leave that for later, and sneak up the stairs to the left.
Head west on the ledge and you’ll come to a broken staircase with a Grave Glovewort  on the edge. If you’re feeling spicy, you can jump the gap north, and then continue straight up the stairs on the other side of the room — otherwise, go back the way you came and head up the stairs. After you go up the stairs, you’ll need to go straight and then turn left to enter the hallway there. This is complicated by the knight archer to the east in the main room that fires both regular arrows and what appears to be Radahn’s Spear. Additionally, a second knight archer waits at the far end of the hallway, and as you approach him, a spirit soldier will spawn.
If you want, you can try to fight the soldier in the room he spawns in, using the walls for cover, but if you aren’t quick enough his knight friend will join him. The other option is to run down the hallway far enough for the soldier to spawn, and then lead him all the way back to the stairs in the main room, where you can use the nearby column to protect yourself from the first knight archer. This means no 2v1, but involves more arrow dodging.
Once the soldier is down, you can then deal with the second knight archer. If you ran all the way to the main room, this guy might be waiting just inside the hallway, so be careful. There’s nothing in this first room where the soldier spawned, so continue down the hallway, turn right, and move into the next room. Here, there are two weak enemies sitting on the ground — take ’em down and grab the Grave Glovewort  before continuing east into the next hallway, where another soldier spirit will spawn.
With another ghost down, you can keep going down the hallway, which will turn right to head south. A soldier hides around the corner on the right in the next room waiting to ambush you, so watch out. In the middle of the room, turn right to look west and note the window there — we’ll jump out of it in just a bit. First, head into the room to the south, where another Grave Glovewort  is growing. You can either fight the knight here, or just sneak up, grab the Glovewort, and then jump out of the afore-mentioned window.
Once you jump out the window, turn left and head south up the stairs. You’ll see four gargoyle imps perched along the ledge and on either side of the doorway on the highground — they don’t seem to aggro, but to be safe, kill them all one at a time anyway. Then jump through the window to the west, where you can get revenge on that pesky knight acher.
Turn left to find the lever for the boss door — pull it, then jump back through the window and turn right into the room full of rot. A Cleanrot Knight apparition will spawn — you can choose to fight it, or just grab the Radahn Soldier Ashes from the corpse and then jump through the window. From the ledge with the lever, you can drop down and run west to go through the now-open boss door and face the Ulcerated Tree Spirit.
The boss described in the guide linked below is identical to the boss in the War-Dead catacombs, except that the one here in the War-Dead Catacombs does rot damage instead of lightning, and drops Redmane Knight Ogha.