Oceiros, the Consumed King is an optional boss fight, but you need to do it to get through to the Untended Graves, and to get the Path of the Dragon gesture that gets you to Archdragon Peak. Oceiros lives at the back of the Consumed King's Garden, has lost all his marbles and talks to a baby that doesn't exist, and has been left to his own devices for a while. This kind of thing seems to be the Dark Souls 3 equivalent of letting a problematic relative live in your converted attic and just leaving them to it until you can never go up there again:
"How come I can't go into the garden?"
"Oh, Oceiros has been living there a while, he's just kind of made it his own now."
"Couldn't we put him in the disused cathedral?"
"Nah, we had to put Aldrich in there when he started eating people."
Ridiculous. Anyway, Oceiros is a pretty easy fight, in the context of Dark Souls.
There's a quick run through to Oceiros' boss area, assuming you've opened up the shortcut along the right side of the Consumed King's Garden. Start from the Dancer of the Boreal Valley bonfire at the High Wall of Lothric and go up the ladder and then up the staircase on the left. You should have opened the door immediately on your right, so run in and through the room there and along the outdoor walkway to the lift. Take the lift down and, about halfway, jump off it. You should see the spot you can do this pretty clearly.
By getting off here you can run along a rooftop, still on the right side of the garden, and avoid all the poisonous ground and giant abyss beasts below. You'll come to a stop behind a robed hollow with his back to you. Kill him and jump down from the ledge he was on. Oceiros' boss fight is through the doorway across the courtyard on your right, guarded by two knights that you can easily bait out and run past. The summon sign for the fight, if you need it, is on your left, through an archway into a circular courtyard. Oceiros does magical damage, but also has an attack in the second phase that can fill your curse meter, causing instant death, so you might want to prepare for that too.
In the first phase Oceiros, who was probably once human and now looks like a blind, squishy dragon hybrid, will fight standing upright and with his staff as a weapon. He'll mostly attack by swiping with this, although he will also high–jump and slam down, and whip with his tail. This phase, though, is easy for both ranged and melee types, because Oceiros is cradling his imaginary baby, and doesn't attack with much aggression yet. This means that ranged players will have longer to get off shots, and melee players don't have to dodge around much (as ever with larger enemies, staying up close and under him is a good place to avoid getting hit for melee fighters). Oceiros is particularly weak to lightning damage, so use any lightning consumables or miracles at your disposal.
The attack to look out for in this phase is an area of effect ice one, telegraphed by Oceiros tapping his staff into the ground. Get away from the ice as quickly as possible, because it will damage you as well as stacking for frostbite. Other than that you should be able knock Oceiros down into his second phase quickly, which is when he gets more annoying.
Once you've gotten Oceiros down to about two thirds of his health bar he either suddenly realises his baby is dead and/or never existed, or he thinks you've killed it. In any case, this triggers the second attack phase for Oceiros. He drops down onto all fours and starts wildly smashing around the room, often seemingly not even targeting you specifically. In this phase it's much harder to hit Oceiros simply because he's moving so much and so quickly, which is why it's annoying.
For ranged fighters try and use attacks that have a short cast time or can home in on Oceiros. He has recovery periods after he smashes through pillars (which is why you shouldn't rely on the pillars for cover too much), and he also stops to perform an attack where he breathes crystals, so you can use those gaps to go for more powerful attacks, as long as you're not standing in front of him. Generally speaking it's easy to avoid damage from range in this phase because he often takes out aggression on the environment in general rather than you directly.
For melee fighters you're going to be moving around a lot more to try and actually hit him. His movements are much wilder and harder to predict, with a lot of random charges and 360 spins when you're up close, and because he's low down on all fours you won't be able to get underneath him. It's wiser to play with a bit more caution, when in the first phase you could afford to be aggressive. Do a lot more dodging and keep your shield up more, moving in and out to get hits in.
Everyone should watch out for the crystal breath attack, however, because that's the attack that fills up your curse meter, and that's a good way to die if you're not paying attention. It's easy to avoid the attack if he's performing it from the ground, and this is also a great time to get some cheap shots in, but in the second phase he can fly for short periods and will breath it down from above as he zips across the room. When Oceiros is in the air you should be moving too, to better avoid any potential attacks.
Overall Oceiros' fight looks more difficult than it is. The worst bit is that he keeps talking to his imaginary baby.
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