Central Gutter
From that bonfire, backtrack a bit and destroy the wooden planks blocking the entrance to the next room. Defeat the undead here and you'll also find a lot of ladders. Don't go down any of them yet; find a couple of jars in the corner and destroy them to get Dung Pie x3.
Head to the misty entrance (not a boss, don't worry) and kill the monster. Destroy the jars to get Poison Arrow x20. Continue along the ledge towards the cave with some mists and destroy the mushroom insects. You now have to defeat the large, gas-spewing insect inside. If you have ranged attacks, take out it out safely from a distance; otherwise, attack only with melee when it is not spewing gas. Loot the Poison Moss from it and you'll also get Poison Moss x3 from the corpse inside the jar behind that giant insect.
Now you have to go down. There are several ladders here; for easier reference, I suggest going back to the entrance of this platform, right before the bonfire. From the entrance, go to the left and find a couple of jars in the corner. Go down that ladder and drop down to the next platform below to get Rotten Pine Resin x3 from the jar.
Look at the edge of the wooden platform and jump down to another ledge with a jar. From that, look again below to find a ledge with a chest. Loot the chest to get a Great Club. Carefully go down to the bottom and kill all the undead. Destroy the jar beside a ladder to get Lifegem x20.
Find a couple more jars in the far side of the room to get Dark Fog and Torch. The cocoon-like things ahead contain some very corrosive acid that will destroy your equipment. Destroy them using a ranged attack then enter the hole.
After dropping off the ledge, loot the body there to get a Fragrant Branch of Yore. Enter the misty doorway (safe) to reach The Black Gulch.