King Arthur lost his sword in battle with King Pellinore. He was severely wounded and so Merlin took him into the forest to a hermit who nursed him. In this forest they came upon a lake. Arthur told Merlin that he needed another sword, and Merlin told him that he should look into the lake.

And in the midst of the lake Arthur was ware of an arm clothed in white samite, that held a fair sword in that hand; Lo, said Merlin, yonder is the sword I spake of. With that they saw a damsel going upon the lake.
- Malory.

Merlin announced that this was the Lady of the Lake. She came to Arthur and told him he could have the sword if he were to grant her a gift when she asked for it. Arthur accepted, and then a barge appeared on the water beside him. The Lady of the Lake instructed Arthur to take the barge and claim the sword.

So Sir Arthur and Merlin alight... and so they came to the sword that the hand held, Sir Arthur took it upon the handles, and took it with him. And the arm and the hand went under the water.
- Malory.