This area, accessed via Blighttown swamp, is on a mound guarded by boulder-throwing Giants. It's the location of a boss - Quelaag - and the second Bell of Awakening. Once you defeat the boss it also serves as the gateway to the Demon Ruins.
Quelaag's Domain Map
(Work in progress!)
NPCs in the area
Full Quelaag's Domain Walkthrough
Coming from Blighttown swamp, go through the entrance cave and you will find a couple Egg Carriers on your way through. They will not attack you and killing them will release some annoying maggot enemies, so its best to simply ignore them. At the end you will find a fog gate and the summon sign for Maneater Mildred if you are in human form and already defeated her in Blighttown swamp. Traverse the fog gate to start the boss battle.
Boss fight: Chaos Witch Quelaag
Once you defeat the boss, go to the back of the boss arena and up the stairs where Quelaag came from. At the top you will find a circular room with a hole in the middle, a door on the right and lever. Pulling the lever will ring the second Bell of Awakening and trigger a cinematic showing the awakening of a giant Golem that pulls a chain to the open gates to Sen's Fortress. Now find the door on right of the room that leads down a staircase to a lower part of the bell tower. Down here there is an illusiary wall, that leads to a secret area with Eingyi the egg carrier blocking the way - don't kill him as he becomes a useful Pyromancy trainer. Talk to him, say 'Yes' to his question and he will get out of the way, allowing you to find a bonfire & The Ancient Spider Queen / Quelaag's Sister. You will need the Old Witch's Ring to be able to hear her talk, but even if you don't have it you'll still get the option join the Chaos Servant Covenant when you talk to her - read up on the covenant before deciding what you want to do - and she can also reinforce your Estus Flask if you have a Fire Keeper Soul. Go back out to the lower floor of the bell tower and there's another hole in the wall that leads down to the Demon Ruins.
With the second bell rung, you can now enter Sen's Fortress.
Notes & Trivia: