After you explore the previous section of the capital related to the Irithyll Dungeon, it's time to move on and fight the boss. Starting from the fire, use the ladder to get to the bottom. Don't stand too close to the edge or you will be hit by the fireball.
Go to the bridge, but beware of the gargoyle that will stand in your way. Since the fight with the creature on the bridge is not a good idea (it is very easy to fall here), use the first attack of the beast (it will rise into the air) to your advantage and run in the other direction. If you want to kill a creature, head back to the stairs to avoid the deadly projectiles. There are 8 Onislayer Greatarrow on the bridge and the Big Soul of a Weary Warrior on the other side.
Upon entering the building, you will encounter a group of enemies on the left side. You can kill them, but you don't have to. As you move to the right, watch out for the gargoyle that will swoop down on you. If you left the mages alive, do not approach them, otherwise they will join the battle (not a good idea, since they shoot fireballs). Kill the creature and you will reach an intersection - if you turn left you will reach the boss, but you can explore the building in front of you and exit through the scaffolding on the right side.
When you find yourself inside (the building in front of you), determine the location of the gargoyle, and only then approach the group of opponents (the creature above, on the right). You must lure the creature out using a bow.
Now you can deal with the rest of the enemies located next to the chests. First, use throwing knives or similar items to separate them from each other (screenshot), as they are best fought alone. Be especially careful with flames that come out of their lamps; if you get too close, you will quickly lose health.
When the area is cleared of opponents, go to the chests on the right (located in the corner), where you will find Ember. The other two are located in the center, and are actually werewolves. Don't fight two of them at the same time. After winning, you will receive two Rusty Gold Coins and a Large Shield of Glory.
Before you enter the arena, go outside. There are three more opponents in the stone ball near the fire. Kill them and move down to come face to face with another monster. Walk to the end and collect the rusty coin. The second coin is located next to the body, just above the abyss.
Don't return to the bridge you came from yet. Find the place shown in the screenshot and jump down. By doing this, you will find the Great Storm Bow.
You have just learned all the major parts of the Defiled Capital. It's time to head to the arena and face Giant Yorm. If he is the third Lord of Ashes, you will teleport to the High Wall of Lothric, where you will face a dying Emma. If not, return to Anor Londo and kill Aldrick.
Remember: if you followed Siegward's quest and freed him from prison, he will help you win the battle. You do not have to summon it yourself: the corresponding cutscene will be shown, and as soon as you enter the arena, Sigward will appear. He can die during this fight, and if that happens, you can take his armor, shield and sword Lord of Storms from his body.
Boss Fight Giant Yorm
