High Lord Wolnir is a giant skeleton with a penchant for fancy jewellery, or, to be more specific, the head, torso, and arms of a giant skeleton hanging on to a steep slope, and you have to kill him in order to progress in the game. That's currently all the information we can give you. Although we did read a loading screen that said something about him stealing the power of some other lords (these may or may not have been skeletons). Anyway. Here's how to kill High Lord Wolnir.
Getting to High Lord Wolnir can be frustrating if you have to do it repeatedly, but it's not too hard once you get the hang of it. Start at the Catacombs of Carthus bonfire and run left and down the stairs, then keep going left, and then right, down the passage on the right at the end of the hall. Keep following the path right and around until you reach the rickety rope bridge. Here you can either run straight across and hope the skeletons don't catch you, or go past the bridge and around the back of the cavern to drop down from a ledge. Either way you'll be across the other side and can go straight on into Wolnir's boss arena.
In order to actually trigger the boss fight you need to touch the skull goblet on the table in the middle of the room. This will then trigger a tedious cutscene. This cutscene is not skippable and must be watched every. Single. Time you attempt the boss fight. This should be taken as extra motivation to dispatch him quickly.
Once the cutscene is over you're standing on a dirt plane with a steep incline, it's pitch black, and basically the only thing you can see is an item orb down the hill in front of you (which you should grab, incidentally, because it's the Grave Warden Pyromancy Tome. It is unclear where in time or space this fight is taking place. Perhaps Wolnir is really a normal sized skeleton, and you are tiny. In any case, once you run far enough down the slope Wolnir will reveal himself.
With the other boss guides we've been breaking it down into the boss's attack phases, but Wolnir doesn't really have any. His attacks are either smashing down with his arm, sweeping across the arena with his arm, or smashing down with both arms at once. Wolnir will also start spawning small skeletons, but he'll end up destroying them himself so you can basically ignore them.
What you need to watch out for in the fight are the just two things. Firstly, he has one fight finisher – wherein he vomits out a cloud of darkness – which can almost instantly drain your health. When he's about to do this he'll raise himself up onto both arms and pause for a bit, so you can run back up the hill and wait for the fog to subside before running back in. Secondly, Wolnir will occasionally skuttle forwards on his hands. Don't back up too far or hang around in the fight long enough that he's pushed you up against the back wall. This makes it basically impossible to avoid point number one.
To kill Wolnir all you have to do is destroy the three bracelets on his arms (so spellcasters might prefer attacks that do area damage, or weapon buffs, because you can't lock on to them directly). The quickest tactic is to aim for the ones on his left arm first (his left; your right) because there are two on that side and if you stay on the outer side of his hand you can avoid a lot of attacks from his other arm. Then when those are out you can run to the other side to destroy the last one. You just need to keep an eye out for the instant kill attack and dodge the slow strikes from his arms as necessary. It's honestly a pretty easy fight as long as you finish it quickly. The longer it goes on the more likely it is that you'll get pushed into the back wall. Which, we repeat, is something you don't want to happen.
Once Wolnir has gone down the door at the back of the room will open, and you're free to walk up and out to look at Irithyll of the Boreal Valley. We'll begrudgingly admit that it's quite a nice view. But the city itself will chew you up and spit you out.
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