Kiln of the First Flame

To gain entry to the final level of the game, you must place the Cinders of the Lords on each of their thrones then talk to the Fire Keeper. Kneel in front of the bonfire afterward and you will be transported to a desolate Firelink Shrine. Go outside and check the coiled sword on the left to get to where you need to go. Move on onto the arena to engage the Soul of Cinder.

Final Boss Battle - Soul of Cinder

This is another multi-phase boss encounter. During the first phase, this boss will shift between warrior class, mage class, and a combination of both. You can tell which class is active based on the weapon he carries - a sword for warrior and a staff for mage. As a warrior, the Soul of Cinder will be doing a multitude of sword combos. Buff your weapon with whatever resin you have on hand. I had to use lightning. In warrior form, dodge the boss's sword attacks and counter at the end of the combos as soon as opportunity allows. I also found that he most vulnerable during mage form, despite having ranged spells available to him. If he switches to warrior-mage form, things can get complicated as he now uses weapon combos on top of spellcasting. Dodge, counter and heal as needed and the first phase will soon be over.

Now for the second phase, the Soul of Cinder becomes a very aggressive melee fighter. He has two main power attacks this round; the first being a four-hit embered sword combo that ends with a ground stab that sends out a fire shockwave. It should be very obvious when he's about to do this, so make sure you're not anywhere near him during that time. The second power attack involves a lightning spear that he throws and comes down on top of you as a lightning AOE damage. One more thing you need to watch out for is he sometimes stabs you and picks you up for a slam. Dodge, counter and heal as needed just like you in the first phase. The battle ends after you defeat the Soul of Cinder here.