Grab the Eye of the Priestess and deal with the Holy Knight Aurheim.
Darks Souls 2 guide - Crown of the Ivory King - Eye of the Priestess
Go down two sets of stairs, along the path and up the stairs to the right. In the room at the top you'll find the Eye of the Priestess, which you need to see Aava, the King's Pet during the boss battle. It allows you to see Black Ice Knights and summon points so you can call in AI assistants to help. You can fight Aava now by following this path.
If you want to continue exploring, go down the stairs and around the corner to the next set of stairs. There's a door under where you found the Eye of the Priestess but you can't open it yet.
Down the stairs you'll see a Black Ice Knight. You can either attack from a distance or get up close. Follow the path to the right and you'll see a Rampart Golem running at you. Fight it. Down the path you'll find five Alluring Skulls. Go up the stairs and through the mist door.
As soon as you go through you'll be attacked by a Retainer Mage on the left. Use the bonfire for cover and kill the mage. Defeat the other enemies here then light the bonfire. This is Frozen Eleum Loyce, Inner Wall bonfire.
Down the opposite hallway you'll find a lever that opens the door ahead. On the right is a small hallway and room where two Retainer Mages and two Frozen Soldiers are hanging out, along with a Frozen Warrior in the hallway on the far side of the room. You can fight the majority of these using ranged attacks.
Move into the room and you'll see a second Frozen Soldier. Take out all the enemies and you'll have to leave the frozen chests for now. When they thaw you'll find a Symbol of Avarice head piece, three Twinkling Titanite items and three Petrified Dragon Bones. One of the chests is a mimic though, so be ready.
Go out the other side and up the stairs to the right. On the next floor the Holy Knight Aurheim will invade, although he's two floors up so not to worry just yet. Instead, take the two Dried Roots in the corner and go up the next set of stairs.
There are two Frozen Warriors and a Retainer Mage here. If you back down the stairs the mage won't follow, so you can thin them out a little. Go back up and kill the mage too.
When this room is unthawed you'll get attacked by an Ice Golem in the middle of the room. Once all three Ice Golem's are dead, you can open the chest for a Curved Nil Greatsword.
You can see that half of the room is blocked with ice. Once thawed, you can get into the hallway and jump down to the first Knight of Eleum Loyce location if you want to follow this route.
Go through the doorway on the opposite side and up the stairs. As you approach the chest at the end of the hallway you will be attacked from behind by the Holy Knight Aurheim. And he's invisible. Be ready or the critical hit will kill you.
You need to see if any of the barrels in the room move - this gives away his location. You'll usually find him next to the barrels on the right. As soon as you hit him or he dodges he becomes visible, so make the most of that opportunity.
When defeated you'll find 10 Silver Talisman items in the chest. Go through the doorway in the middle of the hall, up the stairs and then drop down to the path below. You are now at the entrance where you started. To the left is the boss battle with Aava, the King's Pet. You should head there now and fight Aava.