Move toward the item corpse and the Catacombs of Carthus boss, High Lord Wolnir, will appear directly behind it. You have time to collect the Grave Warden Pyromancy Tome on the corpse. Do so now because you’ll be transported back to the previous room after the fight concludes.
While you can target multiple locations on the head and arms of the High Lord Wolnir, focus on the bracelets on either wrist. It may look like you aren’t inflicting much damage, but as soon as a bracelet breaks you’ll see a significant portion of High Lord Wolnir’s health bar fade away. Get as much damage as you can early in the fight because it doesn’t take long for a group of Skeletons and Skeleton Wheels to spawn and attack.
Luckily the High Lord Wolnir’s attacks will inflict damage on the additional Skeleton enemies, but that doesn’t mean you can simply ignore them. Continue to focus on the bracelets around the High Lord Wolnir’s wrists until a Skeleton gets close, then switch targets and take down the ancillary enemies. Be careful not to back up too far because there is a limit to the size of the boss arena and you don’t want to have you back against the wall, limiting your escape options.
The High Lord Wolnir has a few different attacks. First and foremost, keep your distance from the black cloud that surrounds the High Lord Wolnir’s body. This is essentially a cloud of dark magic that will inflict damage if you get too close. That means you should not be attacking his wrist if it’s too close to his body.
You may also see the High Lord Wolnir emit more dark magic from his mouth. Stay away when this happens, but if one of his wrists is far enough away to keep you out of the dark magic, feel free to attack at will. If the High Lord Wolnir raises either arm, get ready for one of two attacks. The High Lord Wolnir will either slam the arm back down in the same general location, or sweep it horizontally across the front of his body. You can dodge both attacks by simply backing away, but if you don’t want to continually move closer to the back end of the room, simply time your dodge to avoid the attack completely.
As you back away from the High Lord Wolnir it will periodically move closer, once again starting to pin you against the back wall. However, it will back away from time to time. When this happens make sure you run up so that the High Lord Wolnir doesn’t crawl forward again. If you see a small area filled with light, stay away as it’s about to burst and inflict damage if you’re too close.
For the most part this is a fairly easy battle. Just focus on the High Lord Wolnir’s bracelets and you should be able to finish it quickly. When the boss goes down you are transported back to the previous room and you obtain the Soul of High Lord Wolnir.
Light the High Lord Wolnir bonfire, then head up the stairs and through the newly opened door and up the stairs beyond to enter the Irithyll of the Boreal Valley. Light the Irithyll of the Boreal Valley bonfire to the left. With both bonfires lit you can continue on to the Irithyll of the Boreal Valley or head to the side area of Smouldering Lake by traveling to that bonfire.