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Location
Found at the end of the Profaned Capital, accessible through Irithyll Dungeon.

Summons
Siegward of Catarina - will join you automatically if you have progressed his questline to this stage.

Strategies
Storm Ruler
The easiest method for killing Yhorm is to use the Storm Ruler's weapon skill. To do so, you need to hold down the weapon skill button until the storm effect pulsates, indicating that a storm bolt can be unleashed. Press the weapon skill button and the regular attack button to shoot the storm bolt at Yhorm, dealing massive damage.

The Storm Ruler itself is found on the altar behind Yhorm when you enter his boss room.

If you have finished Siegward of Catarina's questline before you engage Yhorm, he will join the fight with his own Storm Ruler, making the fight even easier.

David's return
Yhorm can be brought to his knees for a critical attack by attacking his right hand a few times (about 2-6 depending on NG and weapon type). This can be done easily after Yhorm does an overhead smash or after he finishes a kneeling smash. With that said, Yhorm can be baited to do overhead smashes by going between his legs. However, relying on this tactic means Yhorm may punish players by pulverising them with his machete after he does his overhead smash.

Ranged players may target Yhorm's head and pelt it with arrows for a chance at critical attacks.

Battle
Phase 1
Overhead Smash: brings his machete down from above in a vertical manner.
Pulverize: having brought down his machete, he will use it pulverize anything nearby.
Kneeling Smash: when the player is beneath him, he will kneel down and smash his machete directly in front of him.
Swipe: after recovering from being poise-broken, he will swipe directly behind him if the player is standing there.

Phase 2
Once below 50% health, he will become enraged, taking on an embered look. His attacks all gain Fire damage.
Battle Cry:: he will cry out and taken on his embered form once at 50% health.
Overhead Smash: brings his machete down from above in a vertical manner.
Pulverize: having brought down his machete, he will rock it to pulverize anything nearby.
Leaping Punch: he will leap forward and punch. This tends to be used to catch the player off guard.
Kneeling Smash: when the player is beneath him, he will kneel down and smash his machete directly in front of him.
Swipe: after recovering from being poise-broken, he will swipe directly behind him if the player is standing there.