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."
"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)