“Once Upon a Time” viewers received quite a shock during episode 7 Sunday night. The ABC fairy-tale drama unmasked the first-ever Dark One – and the person had a surprising connection to Merlin (Elliot Knight)!

Episode 7, titled “Nimue,” introduced fans to Merlin’s true love. Nimue was a refugee whose village burned down, leaving her as the sole survivor. She sought out Merlin to help her replant the seeds of flowers that used to grow in the village. With Merlin's magic powers, he could know what a person wanted and see their future -- but it was different with Nimue. The wizard wasn't able to read her or see her future.

Despite Merlin's uncertainty about Nimue, he eventually proposed to the beautiful woman. But he had to confess to her that he had drunk from the Holy Grail and become immortal. He explained that he didn’t want to have to go on living after she grew old and died. Nimue’s suggestion was that she, too, drink from the Holy Grail. However, Merlin insisted that Nimue drinking the magic was not an option. Instead, he suggested a plan B: Melt the Holy Grail into a sword that he could use to remove his magic and become mortal.

It was a romantic suggestion, but not one that Nimue appreciated. When Merlin had his back turned, Nimue drank from the Holy Grail, gaining the same powers as her fiancé. But things took a dark turn when the two were confronted by an evil masked man who wanted the magic – the same man who had burned down Nimue’s village.

Although Nimue wanted Merlin to use his magic to kill the masked man, Merlin refused, explaining that using magic for evil would turn him dark. The masked man took advantage of the situation and grabbed Nimue. She tried to fight him off, but he stabbed her in the stomach, killing her.

Merlin dropped his newly forged sword to hold his dead loved one in his arms -- and it was only then that he learned she had drunk from the Holy Grail. In a surprise twist, Nimue rose from the dead and ripped the masked man’s heart out of his chest. Merlin begged Nimue not to crush it, but she wanted revenge for her village. Despite Merlin’s warning that she’d be lost to the dark side, Nimue used her magic to kill the masked man. A transformation quickly took place, turning her into a shimmery-skinned Dark One. But she was one step ahead of Merlin. She grabbed the sword and broke it so that he could never use it to cut away her magic. However, Merlin still managed to hatch a plan. He used the two pieces to forge Excalibur and the Dark One’s dagger so that he could always have control over Nimue.

In Camelot, Merlin asked Emma (Jennifer Morrison) to summon his ex in order to get her hands on the flame they needed to melt the sword back together. Despite some back and forth between good and evil, Emma did manage to get the flame and the upper hand on Nimue. But King Arthur’s (Liam Garrigan) evil operation with Zelena (Rebecca Mader) unraveled their plan. King Arthur used Zelena’s magic to link the broken Excalibur sword to Merlin – one of the few ways Merlin could die.

Episode 7 concluded in present day Storybrooke with Emma using the flame to melt the broken sword and dagger back together. And although she hesitated for a second before grabbing it, a hallucination of Nimue encouraged her to take it and snuff out the light.

What are your thoughts on the introduction of Nimue on “Once Upon a Time”? Tweet your thoughts to @AmandaTVScoop.