Speak to the Wanderer located in their camp north of Sophanem, which is south of Pollnivneach and over the River Elid. Be sure to have a Cat, Tinderbox and Full Waterskins with you, and be careful of the level 63 Crocodiles near the river! When you talk to her she will overreact to seeing your Cat. Give her the supplies that she asks for, and then she will ask you to look into her eyes.

You will wake up in the new City of the Dead, known as Sophanem, with a Canopic Jar in your backpack. Now touch the door of the nearby Pyramid.

Note: the desert effect will not wear off while in the pyramid, so be sure to take enough waterskins! There is a sink just east of the pyramid entrance where you can refill if necessary.

 

The Pyramid

You will have some sort of dream or flashback, as if you were hypnotized. You'll also find an Unholy Symbol in your inventory. Be careful of the Crushing Walls, and after you're past them, you'll need to watch out for Trap Floors. Don't step where the red X's (on the below picture) are, or you will need to start over again. If you get hit by a wall, it will cause 8 damage.

Now that you're past these dangerous obstacles, you should find some level 84 Mummies, as well as more Crushing Walls. After this, some level 92 Scarabs will appear. Please note that both the Mummies and the Scarabs can poison you for about 2 damage. When you get to a pit, jump across it, and note that you will need at least 15% energy to jump it. When you have jumped across the pit succesfully, go west and try to open the door.

 

You'll have to solve a puzzle in order to enter the room to the east. When you click on a tile, that tile and the 8 surrounding tiles will flip over, revealing parts of the image. Keep clicking on the squares until the image of the bird is completed. You can also reset the puzzle by clicking on the bird to the right.

After solving the puzzle, the flashback will end and you will be back at the Pyramid entance. Speak to the Sphinx, ask him for help, and he will ask you to solve a riddle: A husband and a wife have 6 sons. Each son has a sister. How many people are in the family? The answer is 9, and be sure to get it correct or you may lose your Cat. Now he will ask you to speak to the new High Priest in the southwest part of town. He will give you a Sphinx's Token.

 

The High Priest

After speaking to him, he will tell you to put the Canopic Jar back inside of the Pyramid. Cross into the Pyramid as before, but before you jump the pit you'll have another flashback. Enter the door, and you'll see 3 Jars lined up. Depending on which one you take, you'll get attacked by a different guardian. Try taking the south-most Jar and you'll be attacked by a level 65 Apmekan. Use your Protect From Magic Prayer (if you can), and then take the jar once he is dead (take note of where the Jar was).

 

Try jumping the pit again and you'll get out of the flashback. Jump the pit again, and then resolve the puzzle. Drop the Jar in the same place as you took it from in the flashback, and then return to the High Priest. He wants you to make the spirit rest so that they can be rid of the plague.

 

Spirit Ceremony

Now give your Willow Logs to the Carpenter in the east part of town, and then speak to the Embalmer to the south. He will need:
Salt - use a bucket on the small lake north of Sophanem, go to the southeast-most part of town, use the bucket of Salt Water on the Centrepiece and search it.
Sap - Use a Knife on an evergreen (or any "pointy" tree) while carrying a bucket. Evergreens can be found near the Body Altar, near the Castle Wars Arena, and north of Falador.
Linen - Bought from the Linen Worker in Sophanem for 30gp.

Speak with the High Priest again and he will head to the ceremony spot. Go speak with the Carpenter and he will give you an Unholy Symbol (if you have already given him Willow Logs).

Go back into the Pyramid, make your way to the pit, and you'll have another flashback. Go to the eastern chamber and hide the Unholy Cross in any of the Sarcophagi. Exit the room, jump the pit ,and go to the eastern chamber again. The Priests have already begun the ceremony, but they won't listen to you...

The Wanderer will appear and she will turn out to be the Devourer! She'll posses 1 of the level 91 Priests and cast spells at you. Kill it, speak with the High Priest again, and then you'll have another flashback and meet Icthlarin himself! Leave the Pyramid, speak to the High Priest, and Quest Complete!

Rewards
A Cat Speak Amulet - allows you to speak to your cat! If lost, you can get another from the Sphinx.
4,500 Thieving experience, 4,000 Agility and Woodcutting experience.
Free access to Sophanem
2 Quest Points