Soul of Cinder

Soul of Cinder is the final boss of the game. He is weak against lightning. Cast Tears of Denial prior to entering the boss arena, buff your Lothric Knight Greatsword+10 with lightning, and consume a Red Bug Pellet to boost your fire defense. To complete Yuria's questline, you must summon her to help you fight the boss and ensure that Yuria survives the fight. Also summon Londor Pale Shade if you want. The boss can inflict poison on you; have some Purple Moss Clumps ready.

58. Soul of Cinder, Dark Souls III

After defeating the boss, don't light the bonfire in the arena. You want to remain in the current game cycle, which is NG+0, and do whatever else you need to do such as stocking up on items or farm for items and weapons. Lighting the bonfire would initiate one of several possible endings. A cutscene would play, followed by the credits. You would then be presented with two choices: remain in the current game cycle (NG+0); or proceed to the next game cycle (journey 2). Not lighting the bonfire would prevent you from accidentally choosing the option to move to the next game cycle. In case Yuria survives the fight, return to her location at Firelink Shrine to collect her Ashes.