Speak to King Arthur in Camelot Castle, and he will tell you about an ancient artefact called the holy grail. He mentions that the grail has been moved from England into the realm of RuneScape, and his Knights of the Round Table are currently looking for it.
You offer to help your associates in their quest, and Arthur accepts, asking you to consult Merlin for more information. Merlin, in his office located on the first floor of the castle, believes the grail's exact location maybe known to Sir Galahad, as he was the last knight to come back from the quest, visibly changed. He also mentions speaking to the high priest of Entrana, the last known location of the grail.
Even though Merlin specifically tells you to speak with Galahad, you must visit Entrana prior to meeting him.
Bank any weapons and armour you may have, and take a boat from Port Sarim to Entrana. Alternative methods include travelling via the hot air balloon, using the Abyss to access the Law Altar, or teleport to the Law Altar via the wicked hood.
Speak to the high priest, who is in the main chapel of the church. He discloses that the holy grail, unfortunately, is no longer on the island. While the priest is speaking, suddenly, a strange old crone appears and states that the Fisher King, the protector of the grail, is in trouble!
To help the King, you must travel to his realm using a magic whistle. The crone says that the whistle can be found in "a haunted manor house in Misthalin", although you need an object from the realm to be able to see it. Since the Fisher Realm is a reality different from this one, you need to blow the whistle at the point where it's easiest to cross through. The specific location is found by triangulating the directions in which the 4 mysterious statues face.
Make your way to Sir Galahad, located in a building west of McGrubor's Wood and east of the coal trucks, and talk to him. Tell him you're on a quest for the holy grail, on which he reminisces. Ask him why he left and after some more nostalgia, he reluctantly gives you a holy table napkin, a souvenir he took from the Fisher Realm.
Now head to the top-most floor of Draynor Manor and into the southern room. Pick up the two magic whistles on the table; they will not be visible until the player walks into the room and has the holy table napkin in their possession.
Have the Excalibur, magic whistle, a weapon, armour, and some food with you. Head to where the stone faces point, which is under a tower on the north-west part of Brimhaven. Stand under the watchtower and blow the whistle to be teleported to the Fisher King's Realm.
1. Black Knight Titan 2. Fisherman 3. Stairs to Fisher King 4. Ladder to Holy Grail
You find yourself in a gloomy, mucky area where decay fogs the land. Walk north-west and you'll meet up with the Black Knight Titan, standing on an extremely narrow bridge. The Titan refuses to budge and won't let you through without a match.
The Titan must be killed with the Excalibur blade only, although you may use other weapons to bring down his health. The Titan has no specific weakness to an attack style, but seems largely resistant to ranged attacks, hence it is recommended to use either melee or magic.
Using abilities like Slice, Backhand, Wrack and Asphyxiate can considerably reduce your battle time. He doesn't move, so you can walk away at low health to eat without being damaged.
Once the Titan is at low health, wield the Excalibur and finish him off. If you do not use Excalibur for the final blow, he'll regenerate all his health and you must fight him again.
After you have killed him, cross the bridge and head to the fisherman by the river and talk to him. Ask him on a way to get inside the castle, to which he suggests ringing a grail bell by the entrance. Make your way up to the castle, and ring the grail bell, lying near the door, to enter the castle.
If you didn't get the spare whistle while in Draynor Manor, do so now. There is one on the floor of the castle and one on a table on the first floor.
Head upstairs and speak to the Fisher King. He welcomes you to his realm, saying that the crone and the fisherman were there only to guide you to him. Legends say that one day a "questing knight" would restore the dark land to its former glory, and claim the grail for themself.
The King mentions that he is dying, and wants to meet his son before he departs. His son, Sir Percival, is one of Arthur's Knights of the Round Table. If you do not ask him about his son, you'll have to do it all over again.
Head to Camelot and speak to King Arthur. Though he does not know where Percival is, he gives you a magic gold feather which points to the golden boots of Arkaneeses that Percival was seeking.
The holy grail on the table.
Blow on the feather and it will guide you towards the Goblin Village. Inside the village, head into the east house and open the sacks to find Sir Percival. He sheepishly reveals to getting caught in a goblin trap, after which the goblins bound him in a sack, later forgetting about him. Tell him his father wishes to see him; Percival doesn't know how to get to the castle, so give him your second magic whistle.
Return to the Fisher King's Realm once more by blowing the whistle under the tower north-west of Brimhaven. If you have access to the fairy rings you may also get there with the code BJR. Head back up the hill and go up the staircase on the other side of the castle, climbing all the way up. Pick up the holy grail from the table and head back to Camelot, presenting it to King Arthur.
You can say, "I seek the Holy Grail" to the Fisher King. This was said by King Arthur in the film Monty Python and the Holy Grail. Another reference to the film may be the Black Knight Titan and the armless and legless black knight defeated in the film. Of course, the Monty Python films were based off the original legends of King Arthur and consequently, all references to other parodies come back to them.
The Grail Bell is the same image as the servants image on the construction guide.
If you attempt to take a second grail, you receive the message "You feel that taking more than one Holy Grail might be greedy".
Upon completing this quest, a Player's Adventurer's Log will read: "I defeated the Black Knight Titan and restored peace and happiness to the Fisher King's kingdom. I also found the Holy Grail."
After completing the quest, if you talk to Juna, your character will say, "...And out of all the knights of the Round Table, it was I who found the Holy Grail."