Lancelot returns to Once Upon a Time!

We met Lancelot in the s2 episode "Lady of the Lake". A knight who exiled himself from Camelot for the love of a woman, and who helped out Snow and Charming in the past, and who was apparently killed by Cora (Regina's very evil mother, the Queen of Hearts). Episode 503 "The Siege Perilous" references this when Arthur and Charming talk of the Round Table.

"This is the Siege Perilous, reserved for the knight with the purest heart. The one destined to carry out the most sacred quests. It belonged to a man I trusted more than a brother, but he betrayed me. It's been vacant since."

When Charming tells Arthur that he and Snow met him once, Arthur seems genuinely happy to hear of him and asks how he is. And truly downcast when Charming tells him that he died.

"Lancelot failed to resits temptation but he was a good knight. He tortured himself for his sins more than I would have done and I wished him happy, just not with my wife."

But Snow sees Lancelot at the end of the episode! He warns Snow that the Dark One is not the only villain in Camelot.

"There is a terrible villain in Camelot. (...) Arthur. Trust me, Camelot is not the place it seems."

And with that cryptic beginning, Lancelot is BACK! And definitely not dead. ;o)

Once Upon a Time meets the world of Arthurian Legend in the Season 5 premiere

"Don't. (...) One day you will have the opportunity to remove Excalibur from its stone. And you mustn't. Leave the sword alone."

Says a mysterious cinema usher to a very young Emma Swan. It's Minneapolis 1989. Disney's "The Sword in the Stone" is playing in a theatre. How does this usher know her name? Why does he tell her not to take the sword from the stone? He says if you do something you're not supposed to do, even if you do it for a good reason, bad things will happen. A young terrified Emma watches young Wart pull Excalibur from the stone as our screen fades to black and then to 3 knights galloping across the land: it's King Arthur, Lancelot and another knight! Arthur pulls Excalibur from the stone... an incomplete Excalibur which is missing the tip.

Zoom in on the Dark One's dagger with Emma Swan's name on it, and you realize that dagger looks an awful lot like the missing piece of Excalibur...