Merlin's Crystal

Talk to King Arthur and the knights around the castle. They tell you that Merlin has been imprisoned in a crystal and need you to rescue him. Get some armor and food and head to the docks south of Catherby and have somone distract Arhein while you stow away on the boat.

You will arrive at a castle. Go up both sets of stairs and kill any one of the Sir Mordred. Someone will pop-up asking you to spare the life of their son. Tell them you will if they tell you how to free Merlin. They tell you about a demon. To kill this demon you will need a lit black candle, bat bones, a special spell, and excalibur to break the crystal. To get the black candle get a bucket, and some insect repellent (in house north of catherby bank) and go to the beehives south of Camelot Castle. Use the repellent on the bees and use the bucket on the hive to get wax. Now go talk to the candle maker west of the bank and ask him to make you a black candle. Get the candle and light it with a tinderbox. Now for bat bones, kill a bat (some outside castle with Sir Mordred) and get the bones. Now get the spell, go to the chaos temple in lvl 13 wilderness north of varrock and search the alter for the spell.

For excalibur talk to the lady of the lake (on a peninsula in the lake south of Taverly). She tells you to go to the top floor of the gem shop in Port Sarim. Get some bread and try to open the door, a begger will stop you and ask for some bread, give it to him and he will turn out to be the lady of the lake. She will give you excalibur.

Now go to the pavillion just east of the Camelot Castle. Stand on the pentagon and drop the bones with the candle in your inventory. Say the incantation: "Snarthin/Candletrick/termanto". The demon will die. Now go to the top floor of the east tower of Camelot Castle and use excalibur on the crystal to break it and free Merlin to get your reward.