One of the Grail Maidens will bring you inside the castle
On the northwestern area of Karamja (northwest of Brimhaven) are four pillars. Head in between the pillars and blow on your Magic whistle to be teleported Aion Gold to the Realm of the Fisher King.
Attack and defeat the Black Knight Titan northwest of you (he’s blocking the path across the bridge). Once you kill the creature, follow the river south and talk to the Fisherman. Ask him how to get inside the castle.
Head west and maneuver through the cliffs to the castle. Grab the Grail bell and ring it. One of the Grail Maidens will bring you inside the castle.
Open the door south of you and climb up the staircase. Talk to the Fisher King; ask him why he doesn’t look too well. He’ll tell you that his son, Percival, has not yet returned to rule his kingdom.
To find his son, first head back to Camelot Castle and speak with King Arthur. He’ll hand you a magic golden feather that’ll locate Percival for you.
Go to Goblin Village north of Falador. Head inside the most eastern house and prod on the sack to the north. Then open it. Out will come Percival. Tell him to come with you to become a king.
