The main RuneScape Wiki has an article on: Excalibur
The Old School RuneScape Wiki also has an article on Excalibur.

Excalibur is a magical sword obtained during the Merlin's Crystal quest, it has no known uses outside freeing Merlin from his prison. It is similar in appearance to the Faladian Sword. It can be obtained by speaking to the Lady of the Lake. During Merlin's Crystal she appears in the disguise of a Beggar in Port Sarim outside of Grum's Gold Exchange, asking the player for bread to test if they are worthy to wield Excalibur.

The Beggar's test can be repeated in order to obtain multiple copies of Excalibur using the drop trick. This requires one bread for every copy of the sword obtained. It can be repeated after completion of the Merlin's Crystal and Holy Grail quests.

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