Holy Grail

Ask King Arthur about the quest he is sending knights on, then find Merlin upstairs in the south-east room of Camelot Castle. He appears once you enter.

This is the south-east room, not the south-east tower you climbed in Merlin's Crystal. They are different places.

Sail to Entrana from Port Sarim, banking every weapon and piece of armour first. The high priest says the grail passed through, and an old crone explains the six stone heads and the magic whistle.

Galahad lives in a house west of McGrubor's Wood. Ask for an item from the Fisher Realm and he gives you a holy table napkin. Ask if he gets lonely for a cup of tea and an easy Kandarin Diary task.

Take the napkin to the top floor of Draynor Manor, into the room south of Professor Oddenstein, and pick up both magic whistles from the table.

Take both. One is taken from you later, and going back for the second is a long walk.

Go to Brimhaven with combat gear, west of town and up the northern peninsula to the gold mine, avoiding the poisonous scorpions below combat level 41.

Stand under the guard tower north of the gold rocks and blow a magic whistle.

The Black Knight Titan guards the bridge. He has over a thousand Defence against Magic and Ranged, so fight him in melee — and land the final hit with Excalibur.

Kill him with anything other than Excalibur and he heals to full. He does not move, so you can hit and step back repeatedly with no risk.

Tell King Arthur what you found. He gives you a magic gold feather that points towards the golden boots of Arkaneeses.

Blow the feather and follow it to Goblin Village north of Falador. Open — do not prod — the sacks in the east house to find Sir Percival. You must be carrying the feather.

Give Percival a magic whistle so he can reach the castle.

Return to the realm, where Percival is now king. Go downstairs, up the staircase at the east end, then up the ladder to take the grail, and bring it to King Arthur.