Grab your Catspeak Amulet and a pet cat, then go to Pollnivneach through the Shantay Pass. It's recommended to use the Magic Caprtet just through the Shantay Pass to get to Pollnivneach.
Once there, speak to Sumona. She's in the house directly south of the .
She'll make a request for you to go and save her twin sister, who is down the well in the center of Pollnivneach.
Try to enter the well, and you'll be stopped by Ali Cat.
Ali Cat will tell you that you need to be able to wear a Facemask and Earmuffs at the same time. In order to do this, you'll need to seek out a deceased Slayer Master named Catolax. You'll have to talk to him.
Slayer Master Catolax
For this next part, you'll need your Amulet of Catspeak and cat/kitten still. You'll also need a Ghostspeak Amulet and Rope. Last but not least, bring some gear to deal Ranged or Magic attacks (you'll need to attack from a distance). If you're afraid of possibly dying, you can bring some food.
Catolax was buried in some tombs to the east of the Agility Pyramid. You can take the Magic Carpet to Nardah and go south to get to the Agility Pyramid. Go all the way south, past the Agility Pyramid, until you sea the ocean. Go into the east corner, and you'll see an entrance
Once inside, go to the northwest corner of the swamp. You'll need to push through the reeds that are in the swamp to get through. You'll find a staircase, climb over the fallen pillar in front of it and then go up the stairs. You'll see some archaeologists once you're up there.
You should go west and climb over the low wall. You can use your Rope on a piece of rock on the edge. This will create a shortcut for you to get back here in the future. The Agility shorcut does, however, require level 40 Agility to use. Be careful for the Mummies (you might want to turn on Protect from Melee).
There is a spring near the archaeologists that you can use your Waterskins on. This will let you refill them.
Note: Make sure that you do not have your cat/kitten outside of your backpack before you go into the dungeon.
The dungeon entrance is the first door on the left, it's titled Sealed Door. Search it and you'll go inside. You'll see a small hole in the west wall that your cat can go through. Drop your pet cat in front of it and then search it. You'll ask your cat to take a look.
The cat will open the doors in the west and the east. You can choose either one and go through. Kill the Skeletons that are in whichever room you choose. Next, you'll need to destroy the door to move on. You have to use a Pickaxe to do so. You can get a Pickaxe in the wall that the entrance to the dungeon is in.
Search the Wall, not the Odd Markings, and then run ahead to the next Wall. There are tons of traps in this room, and searching the Wall will disable them for a short period of time. Do this for both Walls, and then open the Door.
Kill all of the monsters in this new room, then turn off your Protection Prayers. Turn your Protection Prayer back on after the Mummy Warrior has popped up from the sarcophagus in the middle of the room. You'll need to kill the Mummy and pick up the key that it drops. Use the key on the door to open it. Go into the next room and pull the lever.
Go back to the first room. All of the traps that you passed on the side you chose have been disabled thanks to you pulling the lever. You now need to go and do the same thing on the opposite side.
Note: if you need to leave the tomb, it'll save all progress that you've made.
Now that both sides are done, go back to the main room.
With all of the traps disarmed, you can just pass over the traps in the central hallway to get through to the ghost of Catolax.
Speak to him, after equipping your Ghostspeak Amulet. You will now know how to combine your Facemask and Earmuffs.
Mistress of the Well
Go back to your bank and create your Masked Earmuffs. You do this by using the Facemask on the Earmuffs.
Wear some dragonhide armor too. You'll probably want to take a full pack of cooked Shark with you since you'll be fighting a level 90 monster and then a level 111 monster. You will still need your Ghostspeak Amulet. You should wear it in order to make room for an extra Shark.
Go back to Pollnivneach and go down the well. Make sure to have your Masked Earmuffs on before you go down it.
Once inside, you'll need to kill a Mighty Banshee for its Banshee Voice. Mighty Banshee are located north of the entrance to the well. Protect from Melee is recommended when fighting this monster. Use the Banshee Voice on the red barrier to the north.
A cutscene will start where you see the Banshee Mistress and her Insectoid Assassins.
After the cutscene is over, you will start your battle with the Banshee Mistress.
When it reaches half health, it'll say "Aid me, minions." At this point, make sure that you turn on your Protect from Missiles prayer. Keep it on that prayer for the rest of the fight.
As with the Banshee from before, use Protect from Melee while fighting it (besides when she calls for aid, as mentioned above). You can use a Summoning familiar if you want to.
Note: If you die during the fight with the Banshee Mistress, your gravestone will spawn right outside the well in Pollnivneach, so you will have time to get your items.
After the Banshee Mistress is defeated, you will see another cutscene. Once the cutscene is finished, head back to Sumona and let her know that your task has been accomplished.
Enjoy your new Slayer Master: Sumona.