Once Upon a Time season 5
Jennifer Morrison as Dark Emma in Once Upon a Time season 5 ABC

Merlin's back story about how he was blessed with magic and immortality will be highlighted in the upcoming chapter of Once Upon A Time season 5. Episode 5 is titled, Nimue, where Emma must summon the darkest force aka the Dark One to forge the ultimate weapon.

The official synopsis reads as follows:

In Camelot, Merlin spearheads a mission to reunite the dagger with Excalibur so he can use the weapon to save Emma from the darkness threatening her soul. With Zelena in tow, Hook, Mary Margaret, David, Regina and Robin steal into Arthur's castle to take the broken sword from the maniacal king. Meanwhile, Merlin brings Emma along on a journey to confront his ancient nemesis and retrieve the sacred spark he'll need to reforge Excalibur. Both parties are tested, but one suffers a blow that could derail the entire operation. In a flashback to long before the Age of Arthur, young Merlin finds purpose after he is blessed with magic and immortality, but when he falls in love with a young refugee named Nimue, everything changes for Merlin, as their romance starts a chain of events that touches every one of our present day heroes, on Once Upon a Time.

Click here to watch it via live stream on ABC, or click here to watch it online on ABC's official website. The new promo shows Rumplestilskin telling Dark Emma about the Excalibur promise. He says, "The Excalibur's promise was born Aeons ago, its time for that promise to be fulfilled."

The voice over declares, "To forge the ultimate weapon, she [Emma] must summon the darkest force of all time", as Merlin and Emma begin their quest to find, the spark of mankind's original fire, which they will require to put the Excalibur together. We see Emma with the Dark One dagger as the name on the sword starts spinning, and a black masked figure appears before her.

Meanwhile, Elliot Knight has teased that the upcoming episode is going to be about Merlin's life. Knight told TV Insider, "I'm excited for people to see more of his story and how he came to be who he is. What is the story that's happened before they've found him in Camelot? You know how he ended up in the tree, but what happened before that? There are so many unanswered questions."

"Yes, I have been very excited for awhile for people to learn some answers to those, and this Sunday's episode is definitely going to be a very big one for answering people's questions," he added.

OUAT season 5 episode 7 airs this Sunday at 8pm EST on ABC Network.