A thousand years before the age of Arthur, Merlin traversed the desert where he was permitted to drink from the Holy Grail. This turn of events is what gave him magic. Two hundred years ago, a beautiful woman’s village was burned down by a masked man, Vortigan. She was the sole survivor. Her name is Nimue. Merlin fell in love with her.
When Merlin drank from the Holy Grail, it gave him immortality. He now wants to forge the enchanted chalice into a sword to cut away this gift so he and Nimue can grow old together. A visit to Nimue’s village shows it in ruins. It’s obvious the Dark One was questing for the Holy Grail. A short time later, Merlin uses magic to turn the chalice into Excalibur. After he does, Vortigan arrives. Nimue tries to fight him, but she’s not strong enough. Vortigan kills her.
Jumping ahead in time, Merlin tells Emma they can make Excalibur whole again. They need a spark from the Flame of Prometheus. They will need to confront the first Dark One, the entity from which all future evil spawned. If they fail in their journey, Merlin will die and Emma will become the most powerful Dark One ever. Hook gives Emma a ring to protect her along the way. He’s not proposing. He just wants her to have something to show that she is also a survivor.
Emma and Merlin arrive at the site where Excalibur was forged and broken. It’s also the place where Vortigan killed Nimue and stole the Flame of Prometheus which is kept in the form of a small ember. Merlin hands Emma the dagger to call on the spirits of the previous Dark One. When the mask is removed, a face from the past is revealed. It’s Nimue. She’s the Dark One.
In the past, Nimue drank from the Holy Grail to achieve immortality. She took the heart of Vortigan and crushed it. After that, she became the first Dark One. She broke Excalibur and left a devastated Merlin behind. Now the first Dark One is forcing the newest Dark One to kill Merlin. Emma is able to fight this. She takes the ember. Nimue warns that she’s not dead yet before disappearing. As for Emma, she admits that it feels good to take the right path.
Merlin explains that he kept Excalibur safe by placing it in a stone with his apprentice. Then he created the dagger to Nimue’s spirit so he could control her reign of terror. But eventually she got it back and put him into a tree. Emma lets Merlin know that she could see that Nimue still loves her deep down. Now it’s time to find the rest of Excalibur.
Zelena offers to help the heroes to sneak into Arthur’s castle. She wants her magic back in return. First things first. The heroes need to get Excalibur back. Inside the castle, Arthur orders that deadly defenses be used on all intruders. Outside, Zelena knocks out Mary Margaret. She joins forces with Arthur. Zelena tethers Merlin to the incomplete sword. Arthur summons him to do his bidding.
Merlin lets Arthur know that Emma has passed her test. He wants him to turn over the sword. Arthur refuses to do so. He orders Merlin to make the heroes disappear from his Castle. In present day Storybrooke, both Rumplestiltskin and Nimue tempt Emma to forge the dagger with the sword. She does. Excalibur is whole once more. Emma remembers Merlin’s warning to her when she was a little girl. She ignores it. She takes the sword. She takes the power.