Boss Battle: Elana, the Squalid Queen
Reward: Soul of Elana, Squalid Queen
As soon as you enter the boss battle area, rush toward Elana and inflict as much damage as you can. She takes a moment before her attack begins, and you can inflict a good amount of damage before her health bar even appears. She summons skeleton helpers and uses various flame-like attacks, as well as her spear when you get close to her.
Once the skeletons appear, back away to avoid being ambushed. Unless the skeletons run directly at you, ignore them for the most part. While you attack Elana, you'll inevitably hit the skeletons as well. Stay on the move to avoid Elana's ranged spells and make your way toward her. Try to stay behind her while you attack to avoid most of her attacks, as well as the skeletons.
She disappears from time to time, and usually uses another spell when she reappears. Most of her spells take a decent amount of time to cast, which means you can hit her once or twice before moving to avoid the attack. Again, stay behind her to not only avoid her spells, but also her melee attacks. If you focus primarily on Elana and continuously move to avoid her spells, you shouldn't have much trouble bringing her down.
Once Elana is down, the huge dragon door behind her opens. Head through it to reach another bonfire. Just beyond is a mist doorway that leads to a large open area in which you battle against Sinh, the Slumbering Dragon.