Head back to RuneScape gold the aviansie in the Eagle's Peak Mountain. They will tell you that your soul is in danger after helping Azzanadra. Describe to them the location of Jaldroacht Pyramid, and they will issue orders to the Desert Eagle underneath them. Next, you'll be told that Azzanadra must be intending to use the Vault of Souls to ressurect Zaros. Its place is unknown, but it will be in a cursed place.
You are told the place is also underwater. You are thrown backward, and several spirits appear. At this point, you must run. Proceed to the aviansie and they will be at a loss for words. Proceed to the Oracle, and she'll shriek when she finds you moved there. She will tell you that only a legend who wielded an elemental power, and has been the master of his own mind and body, ever escaped by the vault of spirits. The Oracle will then cite that though he escaped the vault, he never left the Ancient Cave.
He'll automatically discard some chewed bones. Burn them out on a pyre ship. If nothing happens, speak to Otto Godblessed about it. He will suggest a potion to help. Make a rest potion from spirit weed and snape bud. Use it on your pyre boat, light it, and also a soul will appear and provide you a secret. He'll state you must also wield elemental power and control both body and mind. Go to the elemental workshop to the room which has the zapper seat and across from it's a locked door with the entire body rune symbol on it* Open it with your own key and hunt some shelves for the book of the elemental spear.
Go upstairs and make an elemental spear using 2 primed mind bars. Return to the body space, wield your new spear, and pull on the lever in the center. While battling, your stats will be boosted dramatically. You may wield only mind armor against the Homunculus. When the battle is over, your brain armor will have turned into Elemental Balance Armor. This armor has gentle melee stats. The spear (or, even when you've improved sufficient through barbarian training, hasta) has moderate melee stats, and will auto cast spells as well.
Return to the Ancient Cavern and try to go through this Mithril Door. You will triumph. At the center, the creatures will vanish, and also a phantasm appears. He clarifies the Nightmare Creatures are pawns of whomsoever summons them, so both Zamorakians and Zarosians can use them. The Phantasm has been revealed to be a part of Zaros's soul. He'll attack you, inflicting huge damage if you don't have a minumum of one piece of buy RS gold rotten equilibrium armor, or even the weapon.