The Black Gulch

As soon as you enter the area, enter the hole to the immediate left to find the bonfire. Light it up then destroy the nearby statues before you move one. You'll see some black spots nearby; these contain hiding enemies. These black spots seems to be oil; you can set them on fire using Firebombs.

From there, head to the left to find a chest containing Shotel and Great Magic Weapon. Continue along the main path and be careful of the holes on the left side since there are hiding giant worms there. You can kill them from a distance or just block their attack and kill them up close. Loot the bodies from the jars to get a Large Soul of a Nameless Soldier, Scraps of Life and Torch.

There's another giant worm to the left. Take it out and you'll encounter more black patches. Destroy the jars in the middle and get a Radiant Lifegem from the corpse. Carefully check the small cavern to the left to find a chest containing a Divine Blessing. From that spot, find a narrow ledge along the edge to the right which will lead you to a bonfire. Destroy the jars near the bonfire to get a Pharros Lockstone. Carefully thread the last few steps leading to the boss area.

Optional: Forgotten Key

There's an almost invisible ledge between the holes with the two worms where you can drop off. Make sure have lit the nearby bonfire so you can respawn nearby if needed. After dropping off the first ledge, peek to the other edge on the side of the cliff to find a very narrow ledge with a corpse. Fall down there then drop to the last ledge below that leads a cavern.

There are two giants in the area. If you're a ranged character, you can run past them and find a cavern with a chest. They can't fit in so you can just snipe them and fall back to that position if needed. The chest contains Ring of Giants +1 and Petrified Dragon Bone. Upon killing the two giants, you'll receive the Forgotten Key and Soul of a Giant. There's another room in this area where you can destroy the jar and loot Silver Talisman x4 from the corpse.

The Forgotten Key is used to open a locked treasure room in the Pit of Majula. To access it, purchase the most expensive ladder Gilligan can offer and climb down until you reach the wooden platform inside the pit where the ladder ends. Next, climb the other ladder beside it to reach a room that has a Great Lightning Spear, Witchree Branch and Witchtree Bellvine, and Soul Vortex.

The key can also be used to open the large stone doors. There's one in the first ledge where you dropped off to get the Forgotten Key. You'll find Darkdiver Grandahl inside if you encountered him the first time in the Shaded Woods.

There's also another one in The Gutter. Return to the first bonfire in the area, then head to the first platform after the bridge. Drop down below and jump towards the other platform. Climb the ladder and you'll find another door inside a cave. There will be a lot of poison statues inside so destroy them all to get Havel's Greathshield and Havel's armor set.

Boss: The Rotten

This is a rather easy boss, especially for ranged attackers. The only thing you have to avoid is stepping on the burning oil in the battlefield. Its attacks are very slow and easily avoidable. Melee attackers can immediately counter attack after rolling towards or to the side of the boss. As long as you evade right before its attack hits, you'll get a free counterattack since this boss has a slow recovery time as well.

After the battle, find the lootable corpse past the burning oil. Quickly cross the burning patch of oil and loot the Fire Seed from it; heal up before attempting to cross the fire again. Find the other tunnel and when the path splits, go to the right to find a chest. Open it to get a Sublime Bone Dust. Head to the next room and light up the primal bonfire. Use that to return to Majula.