Oceiros, the Consumed King, is an optional boss in Dark Souls III and you can get to him after unlocking Lothric Castle upon beating the Dancer of the Boreal Valley. Oceiros isn’t a particularly tough boss, and having a shield with 100% physical resistance and a decent stability rating can make this fight very simple.
During the first phase of the fight, Oceiros will behave rather passively, and attacks only with the staff in his hand. His swipes don’t hit that hard, so you can block them easily with your shield. Stay close to him and wail on his body after each swipe he takes. He keeps his left hand close to his body and doesn’t attack with that hand at all, so his left side’s a pretty safe spot to attack. Oceiros will occasionally perform a leaping attack, causing you to lose your target lock. Whenever he leaps into the air, wait for just a split second before rolling away to dodge the impact.
The only time you’ll need to keep your distance from the boss is when he performs his crystal attack. He will thrust his staff into the ground, causing icy crystals to pop up from the floor. Make sure to roll away each time he does this, as the crystals will eat through your health bar very quickly.
The fight will enter its second phase once you get him down to about 60% of his health. Here, Oceiros will do away with the staff and start attacking you with both hands. While his attack combos get very vicious in this phase, the good news is that you can dodge them very easily. As long as you roll to the side once he lunges, Oceiros will end up moving past you, leaving his back completely open to attacks. He also has a tail swipe attack, and you can either roll away from this, or stay close to him with your shield up to absorb the hit.
Oceiros will also leap up and perform a crystal breath attack that can get rather dangerous, as this attack causes Curse. Just like the crystal attacks from Seath the Scaleless back in Dark Souls, this crystal breath can build up your Curse meter pretty quickly unless you roll under him. Whenever you see him in the air, roll forward and under him to avoid getting hit.
Overall, this is a pretty straightforward fight. Just remember to roll to the side whenever he lunges at you in the second phase, and go for hits when his back is facing you. This fight will be over in no time at all.
Be sure to check out our Dark Souls III wiki for more tips and tricks.