Merlin's Crystal

Starting Up
To begin the quest, talk to King Arthur in his castle of Camelot. Ask him if you can join the Knights of the Round Table, and he'll suggest you complete a quest first to prove yourself. He'll tell you that his friend and mentor Merlin has been trapped in a crystal by Morgan Le Faye. While they did manage to find him and bring him back to the castle, they haven't quite figured out just how to get him out. Arthur will tell you to ask the other knights if they have any ideas.

Gathering Info
Go talk to the other knights around Camelot. First you should talk to Sir Gawain and talk to him about Merlin how he got stuck. He'll mention that perhaps Morgan will know, since she created the enchantment. However, the Keep Le Faye is all but impenetrable.

Next go talk to Sir Lancelot, upstairs on the first floor of Camelot. Talk to him and ask him if he has any idea on how to get into the Keep. He'll say something about how the only way to get into the Keep Le Faye is from the sea.

Infiltrating the Keep
Now that you've got a lead, head down the road to the south to Catherby. Arhein will be on the dock just south of the bank. If you talk to him, you'll quickly find out that he does, in fact, sail to the Keep to deliver goods. He's not about to let you hitch a ride, though.

Since he's so adamant about it, you'll have to find another way. Grab some armour and go up between the bank and the candle shop and you'll find a few crates against the back of the candle shop. You can click on them to slip inside and hide. There will be a bit of dialogue as workers come to pack you up and put you in the boat. After a while, you'll end up in the Keep La Faye, at which point you can climb out. (Warning: Do not alter your state in any way (e.g. wield an item) or the conversation will end and you will have to go through the conversation all over again!)

Freeing Merlin
Now that you've arrived at the Keep, go inside and go to the top floor of the place. There you will see Sir Mordred. Talk to him and he will attack you. WARNING: Try not to use prayer during the fight as if your prayer runs out right when you "kill" Sir Mordred you will have to fight him again. Watch out for the Renegade Knights that are wandering around the castle and run up the stairs to the very top floor. Up here you'll find Sir Mordred (level 39).WARNING: It is possible for some one else to come in and attack him, they will not get the 'someone else is already fighting that' you will, and you will have to fight Mordred again. Fight him and just as you defeat him, Morgan will appear begging you not to kill her son. In exchange for sparing him, she will finally tell you how to release Merlin. You will need a number of things to do so:

Bat bones
There are bats right outside the Keep. Just go out the other door on the ground floor to get outside and kill one. NOTE:Take the Bat bones not Giant bat wing. Giant bat wings wont work.

NOTE: If you are a skiller/skill pure, you can simply buy the bat bones from the Grand Exchange.

Black candle
Remember the candlemaker in Catherby? If not, he's in the house just west of the Catherby bank. Talk to him and he'll tell you that he can make black candles, but he doesn't stock any because it's seriously bad luck. However, you can convince him to make you some if you just get the wax.

In a house just north of the bank you can find a bottle of insect repellent on a table. Now head to the west where there's a fenced in area full of bee hives (near the flax field). Use the insect repellent on a hive to temporarily drive the bees away, then use a bucket (there is one that respawns just inside the gate) to get a bucket of wax. Take this back to the candlemaker to get your black candle.

Excalibur
Excalibur is kept by The Lady of the Lake. To find her, go just south of Taverley to the lake there (note: you will have to cross White Wolf Mountain, it is recommended for lower level players to teleport to Falador first, and then travel to the destination, as for players who have completed Fishing Contest, just go underground. You can also teleport to your house if you placed it on Taverley). At the north end is a spar of land that is just out into the middle of the lake. The Lady of the Lake will be here. Talk to her, and she'll tell you that she has the sword, but first she wants you to go to the jewellery shop in Port Sarim.

Along the way, grab a loaf of bread from the food shop (located just south of the fishing store), then head to Port Sarim's Jewellery Shop. As you open the door, a begger will approach you, begging you for some food to feed his starving family. Give him the loaf of bread. When you do, the tramp will change into the Lady of the Lake, who was simply testing the purity of your spirit. Satisfied, she'll give you the sword Excalibur.

Magic Words
Go to Varrock and go to the temple in the south-east corner of the city. In the building there (not in the fenced off area), there is a Zamorakian church with a chaos altar. On it will be the words you need: Snarthon Candtrick Termanto. You must find the magic words yourself or you will not be able to complete the ritual.

Freeing Merlin
Now to free the wizard. First, head back to Camelot, don't forget to take a tinderbox. On the outside of the castle (inside the fence, though), there is a fountain on the north east side of the castle, and to the north east, under a small roof, there is a 12 pointed star in the style of a pentagram. Light your black candle and drop the bat bones (be careful NOT to bury them) on the ground. A ghost named Thrantax the Mighty will appear and you will be prompted to say the magic words. The magic words are: Snarthon Candtrick Termanto. If you get it right, the ghost will obey you and disappear. If not, he will attack you and you'll have to try again. Be careful, though, as he is level 92 and can hit in triple figures. If you fail you need to get a new black candle.

Once the ghost is under your control, you'll make your request to free Merlin and the ghost will do as you wish. Now go back inside Camelot and climb up to the very top of the south-east tower (not the seer crystal in Merlin's room). At the top, Merlin will be imprisoned in a blue crystal. Use Excalibur on the crystal and it will shatter. You can simply use the "Smash" option, but you must have Excalibur in your inventory. Head back downstairs and talk to King Arthur. He will grant you honorary Knightship in the Knights of the Round Table. You have finished the quest. Congratulations!

Rewards

 * 6 quest points
 * Excalibur
 * A replica of Excalibur will be placed in the Varrock Museum as a Varrock Museum display by talking to Historian Minas.

Music unlocked

 * magical journey

Required for Completing

 * Heroes' Quest
 * Holy Grail

Trivia
Merlins Kristall
 * Upon completing this quest one's Adventurer's Log will read: "Merlin was trapped in a crystal by Morgan Le Fay, but I freed him using an ancient spell. I am now a Knight of the Round Table."
 * Thrantax's chathead was not updated with the HD update, as with most ghosts.
 * This quest is the only quest that has 6 quest points as one of the rewards. It is unknown whether this was done intentionally, but there is no evidence either on Jagex explaining why they did this.
 * Dropping the bat bones while summoning Thrantax does not drop the bones, they stay in your inventory.
 * You can smash the crystal even when you don't have Excalibur equipped.
 * After the quest is completed, Sir Mordred can be killed with no interference by Morgan Le Faye.
 * If you use the Orb of Occulus whilst in the crate and move the view around, you can see the Fire Obelisk and part of the ice tunnels beneath the White Wolf Mountain.
 * This quest at one point was not able to be completed by skillers due to an update that forced players to wield the sword to smash the crystal, due to the smash option skillers are now able to complete the quest again.