Here he is: the final boss of Dark Souls III. Be prepared for a long fight. The Soul of Cinder has two health bars, and he changes his attack patterns very frequently in the first phase, so this fight may take a bit of practice before you get comfortable and familiar with his patterns. Most of his attacks are fire-based, so equipping fire-resistant armor and equipment will help you out a bit in this fight.
At the start of the first phase, the Soul of Cinder will attack you with a fire longsword. His attacks are very swift, and his patterns consist of mostly regular slashes and thrusts. You can block them with your shield, or roll out of the way. After depleting his health a little, he’ll switch to using a staff where he uses spells like Crystal Soulmass, Crystal Soulspear, and Crystal Greatsword. His staff phase is the best time for you to stick close to him and start dealing damage, since most of these attacks are long range. When you see him charge up the Crystal Greatsword, roll out of the way to avoid damage, and then continue the assault.
The Soul of Cinder also has a lance phase where he will charge at you with a thrust, or perform an overhead swing. These attacks are fairly easy to read, though they can deal quite a bit of damage, especially if you get caught in a thrust. He’ll also switch to pyromancy attacks, where he’ll use Power Within, Great Fireball, and attack you with a scimitar-like weapon. Be careful when you see him charge up the Great Fireball; even if you’re behind him, he’ll turn around very quickly to throw the Fireball at you, so you’ll definitely want to start rolling once you see that Fireball getting prepped.