Now you've got the Eye of the Priestess, head back to the Outer Wall bonfire. When you get there, make sure you've equipped anything that can improve your magical defences. If you have a shield that can absorb 90 percent of magical damage or more then you'll be in great shape. Equip a Spell Quartz Ring if you have one too.
Go down the path until you come to a summoning point. Use it, then go through the big door in front of you. Head left towards the next summoning point, and bring in another assistant. Now go straight through the misty door in front of you.
Aava is waiting on top of a ledge at the end of the path. Depending on how bad the snowstorm is, you may find it tricky to spot the creature, even with the Eye of Priestess in hand. Either fire a shot at the enemy if you can spy it, or inch closer into the area to grab its attention.
You've got a few different ways of handling this fight. Aava can throw some long-range firepower your way, and she also has a few meaty close-range attacks too. Your allies will certainly assist you during the fight, but they may not be able to get aggro off the creature. If you're fighting up-close, you must get away very quickly when Aava pauses and enters a poise stance, as this telegraphs that a powerful melee attack is on its way to your face.
As for long-range attacks, there are two to be aware of. From mid-range, Aava will summon up some crystals which fire bolts of magical damage at her target once they've all lined up. You can either block these shots altogether, or get hit by the first bolt, which will cause the others to miss. When the bolts have finished firing, pillars of flame will erupt - avoid these at all costs.
The second long-range attack involves Aava placing a handful of spots on the ground which erupt to form those same flame pillars. Again, stay well away from them and wait for the flames to dissipate.
From very long-range, Aava can charge directly at you, then finish with a brutal bite that you can't shield away. Should you spot Aava running at you, be ready to dodge away from danger immediately. If you're not at full health, one solid chomp can be enough to finish you right there and then.
Overall, your best approach is to fight Aava at close-range unless your helpers have the creature's attention - in which case, fire away from a position of safety. If she switches focus back to you, run in and get close once more.
When you are fighting up-close, get to the rear of the best and hack away at its rear legs. Stick to one or two attacks each time unless you have a very fast weapon, as Aava can turn around quickly and claw your face. Aava does zip around the place quite a bit, so you'll need to juggle your positioning constantly to finish the fight.
Once the battle's over, go through the opening over on the far side of the area, then enter the smallish room you come to. Go upstairs and then walk through the door when you reach the top to enter Grand Cathedral.
All of our Crown of the Ivory King boss guides can be found from the first page of this guide.