Merlin's Crystal

Starting Up
To begin the quest, talk to King Arthur in his castle of Camelot (ground floor). 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 how Merlin got stuck. He is found in the same room as King Arthur. 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!)

Ritual components
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:If you are doing the quest Rag and Bone Man, bats will also drop Giant bat wings. These will not work for the quest, only the normal bat bones will.

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.

Go a short run north to the bee hives and then east into Catherby. He is in the first house you will come to (in the southwest corner of Catherby). Talk to him and he'll tell you that black candles are traditionally very bad luck to make. You try to coax him into giving you one and eventually agrees—if you will bring him a bucket full of wax.

In a house just north of the bank you can find a bottle of insect repellent on a table. Take it and head to the west where there's a fenced in area full of bee hives and a flax field (access is through the gate on the west end of the connected flax field). Use the insect repellent on any hive to temporarily drive the bees away, then use a bucket on it (there is one that spawns on the south end of the east fence of the apiary or you can buy one from Arhein) to get 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 southeast of Taverley around the lakeshore and just past the balloon travel point. Go out on the peninsula there and near it's end you will find The Lady of the Lake. (note: you will have to cross White Wolf Mountain only if you're coming from the west, 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 through the underground tunnel from north Catherby. You can also teleport to your house if you placed it in Taverley). Also, you can use a games necklace to teleport to Burthope, then run south. 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.

If you don't already have one, grab a loaf of bread from the food shop (located just south of Port Sarim's fishing store) then head north to Port Sarim's Jewellery Shop. As you open the door, a begger will approach you and ask 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. Now satisfied that you are worthy, she gives you the sword Excalibur.

Magic Words
Go to the Zamorakian church in Varrock, located south of the eastern Bank and Aubury's Magic Shop. Right click on the altar and select the "check Chaos Altar" option. The inscription on it reads: Snarthon Candtrick Termanto.

Note: You must see the magic words for yourself or you will not be able to complete the ritual.

Freeing Merlin
Now to free the wizard. Go back to Camelot (don't forget to take a tinderbox with you, the Dwarf army axe will also work). Then, go to the northeast corner of the castle grounds (outside the castle, but inside the fence). Just northeast of the fountain there you will find a small stone gazebo with eight columns. Inside, on the ground, there will be a twelve pointed star, formed by the outlines of four red triangles. Light your black candle within this structure and drop (NOT bury) the bat bones here. 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 will need to get a new black candle.

Once the ghost is under your control, you can ask for it to free Merlin from the crystal and your request will be granted. Now go back inside Camelot castle and from the southeast corner of the ground floor climb to very top of the tower (not where the seer crystal is in Merlin's room). There you will find Merlin imprisoned in the blue crystal. Use Excalibur on the crystal and it will shatter releasing Merlin. You can simply use the "Smash" option without equipping the weapon, but you must, at least, have Excalibur in your inventory. Head back downstairs and talk with 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
None

Required for completing
Completion of Merlin's Crystal is required for the following:
 * Heroes' Quest
 * Holy Grail

Trivia
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 * The quest once was a requirement to teleport to Camelot, but was changed due to lower levels dying at White Wolf Mountain.
 * 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."
 * 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.
 * Upon talking to King Arthur for the first time, he says 'Back in England, Merlin got himself trapped...' This is the first time a quest incorporates a non-fictional country into the game.
 * If you try dropping the bat bones in the gazebo after talking to Thrantax the Mighty, you won't drop them, but it will say in the chatbox "You can sense a powerful presence here."
 * Despite its low difficulty, it rewards 6 quest points, the only quest in Runescape to do so and give the 2nd highest quest point reward, only surpassed by Recipe For Disaster, which gives 10. This is most likely because the quest points are the only reward from this quest, minus excaliber.