Part 20: - Irithyll of the Boreal Valley Part 2
Sully is the dick who sent out Vordt and the other Outrider Knights, one of which we've seen at the entrance to the Road of Sacrifices. According to the description of his Soul he also feeds gods to Aldrich. Sulyvahn is actually a sorcerer and the recently released DLC gives us a tiny little bit of extra lore concerning his young sorcerer years. But we'll get to that.
When the game was first released a lot of people seemed to view Sully as the Ornstein & Smough of Dark Souls III in terms of the difficulty spike he provides. Now this line of thought has mainly vanished. I didn't really find him very tough myself but he is one of the few bosses who get a kill in on me in almost every playthrough. In terms of gameplay he has a few similarities with Fume Knight from Dark Souls II. Much like him Pontiff Sulyvahn is using a bigger and a smaller sword and the backwards slashes he can do with both of his weapons is a lot like a similar move from Fume Knight.
Depending on who you ask the first phase is either the easier or harder one. He is fairly aggressive and will punish you if you turtle up behind your shield by just hitting you until you can't block anymore, so getting comfortable with dodging his attacks is quite important. He can also be parried, if you're not like me anc forget that you even have this move. Roll attacks work really well on him, when using slower weapons you may deal a majority of your damage with them.
In phase two he, like most bosses, gains a few extra move. Mainly he can do magic attacks. One of them is a Soul Spear the other is more like a wave. Additionally he can Jump up and and drop on you, causing a big explosion in the process.
But the main difference is that he can summon a clone. Whenever the clone is out he becomes less aggressive. Is clone will attack first and Sully will follow with the same attack, so expect to dodge the same attack twice every time. Getting all up in his face can help here, as it can interrupt either the clone or Sully himself, saving you a few attacks to dodge, though, you need to be aware of what the other is doing as you attack. If you find him easier without his clone you can just kill it, but he'll summon it again after a short time. You can use this to your advantage by just attacking him while he summons is phantom, as he's defenseless while he does it.