Phase One — The Bulwark King  

Shield and greatsword, and he knows how to use the shield. Almost nothing gets through the front of his guard, so the whole phase is one question: go round it, or break it. Six clean hits on the same raised shield and it comes apart, and the window that opens is the best one you will get all fight.

Greed is punished on purpose. Hit the guard often enough and he stops blocking and answers — he was waiting for it. He drags you out of safety with a chain, leaves the ground to come down on you from above, splits the floor in a lane you have to step out of, and throws his own sword when you think distance is an answer.

Phase Two — The Stormblade 

“The crown was always a cage. Let the storm speak.”

The shield goes, and with it the patience. He closes distance instead of holding it: a gauntlet combo that scoops you up and carries you down the hall, blinks behind you mid-swing, and splits into copies that fight the way he does — they die to one hit and they hit like he does, so ignoring one is not an option.

He also calls the weather. Inside his blizzard the arena shrinks, his roster narrows to swordwork, and every blow he lands feeds him.

And once, he raises a spire of ice out of the floor, forges his blade into a crossbow at the top of it, and empties a volley into the room before coming down on you with the thing he makes next.

The Half-Phase — Twin Blades 

Short, and the only stretch where he defends nothing at all. The sword comes apart into two, the guard is gone, and both hands swing. Trading with him here is not a race on health bars — he drinks from every hit that lands.

Phase Three — The Hollow Magus 

“You see it now. There is no king left. Only winter.”

He never touches the floor again. A wand of ice is drawn out of the room's own cold, rings of crystal turn around him, and the fight moves to range: seeking shards, a beam he visibly breathes in before he lets it out, spears thrown from a cloud of his own making.

Melee is the counterplay now. He has nothing left to turn a swing with, so the answer to a caster you cannot reach is to get close and stay there.

And if you take him to the edge, he does not go quietly.

“If I must end, then everything ends with me. THE LAST WINTER FALLS.”

The Sovereign's hoard  

• Sovereign's Lament — his blade. Every hit inflicts Frostbite; hold and release for Winter's Sorrow, a ring of ice erupting out of the floor around you.
• Crown of the Hollow King — immunity to freezing and to Frostbite. A cage, reforged into a keepsake.
• Heart of the Silent Winter — consume it once for two permanent hearts and permanent cold immunity. It is still beating.
• Everfrost armour, forged from Everfrost Ingots. The full set carries Rimeguard: blows from the winter charge the plate instead of breaking it.
• The Silent Winter — a music disc.

All of this is in the mod already:

• The fortress — 88×72×88, sealed walls, one way in, chambers inside the walls, vaults, a crypt and a throne cathedral at the end of it.
• The court — Frostbound Sentinels, Rimeweavers, Stillbows, Frostmaws and Vault Wardens hold the halls, and hold them harder while the Stormcrown is still standing.
• The Ice Monstrosity — a second boss and a completely different problem. Six blocks of walking ice with a trough of meltwater across its back and a jaw that never quite shuts. Velkhar can call it.