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)

In episode 504 "The Broken Kingdom" we see how the whole story between Lancelot and Guinevere came about. No longer a happily in love couple, as Guinevere was pretty much abandoned by an Arthur completely obsessed over finding the Dark One's dagger, believing he couldn't truly build Camelot and rule as its true king without it. Forgetting even his wife's birthday, Lancelot organized Guinevere's party in Arthur's name (and of course she sees through this). And when she heads off on her own quest to find the dagger and end this sorry state of affairs, he goes with her... and almost dies in the Dark One's vault. Guinevere - realizing she almost lost him - kisses him and then snaps out of it saying that will never happen again.

The Quest leads them to the dagger, thanks to Merlin's gauntlet (magical item Guinevere found in his tower that leads her to Arthur's greatest weakness), but they can't retrieve it, stopped by a magical barrier and Rumplestitlskin. He proposes a deal, in exchange for the gauntlet he'll give them magical sand from Avalon which can "fix" anything, including Excalibur, and Arthur need never know it's not the real deal. 

"What's one little secret, if it means getting your husband back. Assuming of course that's still what you want."

Upon returning to Camelot, Arthur sees them saying a cozy farewell and confronts her. He goes nuts when she says she found the dagger but couldn't bring it. She tells him he has to stop trying to fix the dagger and save his marriage and his kingdom. 

"Guinevere: You have to make a choice. Stop trying to mend that sword, or your marriage - and your kingdom - will be broken forever.
Arthur: And what if I can't?
G: Then I'll follow my heart, to where it belongs."

He says he can't choose, and can't lose her to Lancelot, so he uses the magic sand to enchant her by "fixing" their marriage. She appears to be a bit brainwashed and says she hasn't supported him enough in his quest. He says she deserves to live in the kingdom she deserves until then, and uses the sand to "fix this broken kingdom" (and basically magically builds the castle we've seen).

In the present time, Snow and Lancelot take the dagger to the Dark One's vault to hide it, but are found by Arthur. To save Lancelot's life Snow gives Arthur the dagger, and he reveals his true colors:

"I have waited a lifetime for this. Now I control your daughter. I can command her to reunite the sword and free Merlin. Excalibur's blade will be the last thing he sees before I run him through. Then I will use the blade to snuff out the Darkness once and for all. I can become what no one thought I could, the greatest king in all the realms."

But when he tries to summon the Dark One, it turns out it's a fake dagger! And Charming shows up to confront him as well.

"Arthur: You betrayed me, you broke the sacred trust of the Round Table.

Charming: You tried to trick me with a catchy title and a comfy chair

A: (...) Every choice I've made was for the good of Camelot. I'm trying to destroy the Darkness!

C: The Darkness is in my daughter, so every choice I make is to protect my family."

Previously Snow had asked Lancelot what happened between him and Guinevere:

"Lancelot: It's complicated, but what's happened is in the past. It doesn't matter anymore.

Snow: It does if you still love her. Do you?

L: Yes. I'll forever love Guinevere."

He doesn't know she was ready to leave Arthur and follow her heart to be with him, but when she shows up to free Arthur (captured by Lancelot, Snow and Charming in Grannie's), Lancelot realizes Arthur must have used the sands of Avalon on her. And then they use the sands on Snow and Charming to "fix" the friendship so they can keep working together.

Locked up in the dungeon Lancelot meets Merida who tells him

"An enemy of King Arthur is probably a friend of mine."

What on earth is she doing in the dungeon and how is Arthur her enemy? In any case this brings with it a promise of some interesting adventures for Lancelot, if he gets out of that dungeon alive that is... ;-)

Elements from Arthurian Legend

• Arthur, Guinevere, Merlin, Excalibur, Camelot
• Lancelot and Guinevere romance
• Similarties: A married Guinevere falls in love with Lancelot, realizes it in a moment of danger, but will stay with Arthur out of duty and in memory of the youthful love they once had. 
• Differences: Here it didn't reach the point of full-blown adultery, although she clearly loved him enough to be willing to leave Arthur for him. And here it was Lancelot being in danger that sparked the kiss, not Lancelot rescuing Guinevere like in the traditional tales.