Excalibur

Excalibur, previously known as the Holy Grail, is the mightiest sword in all the realms, powerful enough to vanquish any enemy.

Before the Curse
A man named Merlin finds the Holy Grail and drinks from it, and it gives him magical powers and turns him immortal. Centuries later, he falls in love with Nimue, a mortal. Wanting to grow old with her, he decides to cut himself from his magic so he can be mortal again. He uses the Flame of Prometheus to reforge the Holy Grail into the sword Excalibur. However, Nimue previously became immortal after drinking from the Grail and later becomes dark after killing Vortigan. Fearing Merlin will take her powers, she smashes Excalibur, severing the tip from the blade. ("Nimue") To keep Nimue's darkness from destroying the realms, Merlin turns the tip of the sword into a dagger and binds her to it so she can be controlled. He places the other part of the blade in a stone, which can only be pulled by the kindom's true ruler. ("Lost Girl"/Nimue")

Centuries later, a young boy by the name of Arthur begins getting dreams, prophesying him to become the king of Camelot after pulling a sword from a stone. When Arthur grows up, he and his fellow knights manage to track down Excalibur. After witnessing it disintegrate Sir Kay, Arthur steps forward and successfully pulls it from the stone...only to discover it is missing its tip. He and his knights decide to lie and the quest to find the missing piece begins. Arthur returns to his village to show them Excalibur, but he hides the broken tip. When Prince Charming feels like Snow White has lost herself, he uses a fake Excalibur to convince Snow into believing she is the true ruler of her kingdom so she'll stand up to the Evil Queen. Arthur's quest to complete Excaliubr spirals him into madness, leading his wife Guinevere to retrieve the missing half. Instead, she returns with magical sands that make anything appear fixed. An angry Arthur ends up using the sands on his wife and kingdom.

Season 5
As the residents of Storybrooke stand beside Granny's Diner in the woods, which they used to travel to the Enchanted Forest, King Arthur, who is carrying Excalibur, and his knights, including Percival, arrive on their horses. Protecting her friends from potential danger, Emma asks who they are, so Arthur introduces himself and explains that they've come to find them. A confused Hook asks what he means, so Percival tells Arthur they think their arrival a surprise. Snow asks if he was expecting them, so Arthur explains that Merlin their arrival a long time ago. Emma questions where the wizard is for she was told that he's missing and Arthur confirms that he has been for years, but not for much longer, because according to the prophecy, they're destined to reunite them with Merlin. The king then escorts the group back to Camelot, taking them all inside the castle where they're welcome by trumpets.

Before the Third Curse
TBA TBA  TBA  TBA  TBA ("Nimue") TBA  TBA