Start by talking to Erjolf outside the cave, who will have found an apparently boring cave. You'll catch him though and he will let you see what he found if you promise to keep it a secret.
Enter the cave and he will call across to you from the other side of the stream. Jump across at the flat rock. Follow him into the cave opening.
Talk to him again, and he will tell you about his Fremennik Trials. Offer to help him with his trials. He will show you how to make and light a fire raft.
Melting the Ice
Block the south and west tunnels by going to them and shoveling snow from the snowmound. Now, go to the spots shown on the west side of the cave and light a fire raft.
Now open up the south tunnel and block the east. Place the fires in the following locations on the north side of the cave. For the last image, set off four fire rafts as quickly as you can.
Finally block the south and open the west. Same with the last group, light four fires quickly in a row at the spot marked.
Go talk to Erjolf again. He will want a trophy from the monster, but doesn't know why the ice around it hasn't melted. He tells you to go find the natural historian at the Trollweiss Hunter Area.
Ask if he is any good at identifying creatures. Describe the following to him:
- It's yellow.
- It has four arms.
- It has no legs, like a snail.
- It has spikes, which run down its spine.
- It has a spiky tail, with vicious spines.
- It has pincers, like a crab.
He will tell you it is a mythical creature. Ask where you can find out more information. He will tell you to search for a statute in the desert east of the Desert Mining Camp.
Look for a large pile of sand, and uncover the statute underneath it. Ali the Wise will show up. Talk to him and tell him you've found a creature just like it. Keep going through the conversation and he will tell you how to melt the ice.
Return to the cave with 8 cosmic runes, 8 water runes, and either bring your own 4 cut sapphires, or you can mine some in the cave near Erjolf.
Talk to Erjolf and explain the situation, then cross the floating bridge. Click on the encased Muspah to bore the sapphires into it. Then enchant them. The ice will melt and Erjolf will come across and poke it. It's alive!
Talk to the Muspah and you will remind him of where and what he is. He will finally realize and transform into the Mahjarrat Jhallan. Go through the options and he will tell you his story. You will agree to help him hibernate again under the Ritual site.
Exit the cave and talk to Erjolf. He will suggest making a canoe. Meet him at the western side of the Trollweiss Hunter area.
Talk to Erjolf there then make the canoe. Travel with it around the coast and enter the nearby cave. Look for Jhallan inside the cave and talk to him. Lead him all the way through the southwest end of the cave and talk to him again. Bore the sapphires around him and enchant them once more. Before you do though, you will talk him into giving you a trophy for Erjolf.
Exit the cave, take the canoe, and talk to Erjolf again to finish up.