This boss is called Yhorm, and he is a giant. Nah, but seriously, he's the guy from the trailer that sat up and did a big scream and started to set on fire, and everyone was really excited. Remember that? Yhorm is the area boss for the Profaned Capital, as well as being a Lord of Cinder. From reading item descriptions and so on it seems he was probably an alright sort of chap at one point, but he's not any more.
1. The Approach
Yhorm's boss area has one of the most frustrating run throughs of the game because there are a lot of enemies on it, so it's difficult to get there without taking damage. From the Profaned Capital bonfire go down the long ladder on the edge of the tower. In front of you is a stone walkway, and from here you have two options. You can keep going straight forwards towards the gargoyle and try to roll under him when he leaps to attack. Then, as you get inside, run right and then left into a passage that leads to the boss arena.
The other option is to drop off the walkway to the right, jumping down to successive ledges until you reach the ground. You can then run in a straight line, up the stairs towards a burning altar, and then into the boss arena.
In either case you will, sadly, be having to deal with the occasional fireball being hurled at you from the right side, as there are a bunch of creepy looking robed casters standing around the aforementioned burning altar.
2. Yhorm the Giant's first phase
When you enter Yhorm's throne room he'll be sitting there at the other end, looking stoic, and won't start moving until you're about a third of the way across, at which point he'll start very slowly standing up and moving towards you.
However, when the fight starts you don't need to be thinking about engaging him. Your job right now is to get to the opposite end of the room and getting the item orb next at the foot of Yhorm's throne. You just picked up Storm Ruler.
Storm Ruler is a sword that for some reason does a huge amount of damage to Yhorm - and only Yhorm - when you use its Skill. It takes a few seconds to charge, which is when you'll be most at risk, but once it is charged you can lock onto Yhorm, run up to him, and unleash the charge with a light or heavy attack that does massive damage (you can take him down with four heavy attacks). Once the weapon has been charged it stays that way, too, so you can dodge about as much as you like until you're ready to use the attack. The best part is that Storm Ruler doesn't have any stat requirements for you to use it, so you can get in on the action even if you're not a warrior or tank type.
As is befitting a giant his movements are slow and pretty easy to avoid, but will do a lot of damage if they hit. He's wielding a giant razor/machete type thing so has longer reach than other giants you may have fought or avoided fighting, but the principle remains the same: hang about around his feet and very slightly behind him, because his attacks very rarely hit there. You'll most likely get your opening to charge the weapon, or attack with it, when he's completed an overhead smash into the ground, as he has to pause for a few seconds.
3. Yhorm the Giant's second phase
The same as the first phase, but Yhorm is on fire.
4. Fighting Yhorm the Giant with Siegward of Catarina
This is a special bonus technique for anyone who has followed Siegward's storyline so far, meaning anyone who fought the fire demon with him at the Undead Settlement, freed him from the well at the Cathedral of the Deep, talked to him in the kitchen at Irithyll of the Boreal Valley, and unlocked his cell in the Irithyll Dungeon. If you did all of these things then Siegward will turn up to fight Yhorm with you - you won't even have to summon him, he just arrives. Siegward makes the fight substantially easier, because he has a Storm Ruler sword too. You can basically sit back and watch him do it for you.
Once Yhorm is dead you can collect his soul and light the bonfire. If he's the third Lord of Cinder you've defeated you'll be transported right on out of there after a cutscene. Otherwise it's time to go back up to Anor Londo.
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