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.