Nimue

In this episode, we finally learn the identity of Nimue as the flashbacks reveal the origins of Merlin's magic over 1000 years ago (drinking from the Holy Grail after asking permission from God) and the first Dark One. After 500 years of immortality, Merlin falls in love with Nimue and is willing to give up his magic for her, although she is intent on using the Holy Grail to become immortal herself and wants revenge on Vortigan, the man who destroyed her village and family. They agree to search for the Holy Grail so it can be forged into a sword so that it can get rid of his immortality, only for Vortigan to arrive looking for it himself. After they fashion it into Excalibur using the Flame of Prometheus, Vortigan arrives and seemingly kills Nimue, only for her to reveal she had drunken from the Grail and ends up ripping Vortigan's heart out, with the darkness in his heart infecting her after she kills him, making her become the first Dark One. And to add insult to injury, she breaks Excalibur in half to prevent Merlin from destroying her new magic, which leads to him putting the sword in the stone and fashions the broken piece into the dagger to control her. In the present Camelot scenes, Emma & Merlin decide to find the Flame of Prometheus to put Excalibur back together, although it requires Emma to summon the first Dark One as Nimue still possesses it. And despite Nimue's taunts to kill Merlin and convince her that she needs the power, Emma succeeds. Meanwhile, the others try to find a way to obtain Excalibur, with a little help from Zelena, only for it to be a ruse so Arthur can summon Merlin as he's tethered to it now. Despite Merlin's pleas, Arthur refuses to hand over the sword to him so he can keep his 'glory' & sends the group away. In Storybrooke, Emma finally succeeds in putting the dagger and sword back together, which cements her now as the most powerful Dark One ever to have lived.
This episode is without a doubt one of my favourites in the show. Why? Because the twists, turns & mythology of the Dark One are all executed perfectly. The flashbacks were so compelling from start to finish, rather it be seeing the origins of Merlin's magic, Nimue's motivations for desiring revenge and immortality, or the origins of how Excalibur came to be. I had been waiting to see Merlin's backstory and the origins of how the Dark One came to be for quite a while (especially the latter), and blending the two together along with the incorporation of Prometheus was a match made in heaven, with Elliot Knight and Caroline Ford's performances being spectacular. While by the end it was a bit predictable that Nimue would be the first Dark One because of the obvious red herring Vortigan, it was still very well done because while she easily succumbed to revenge and power, she still had her sympathetic side. The Camelot scenes with the Storybrookers were also executed very well as Emma's development keeps on growing & I really enjoyed seeing her bond with Merlin during their quest to find the Spark of Prometheus. Jennifer Morrison's acting was amazing during her scenes with Nimue ('I'M NOT NOTHING! I WAS NEVER NOTHING!', anyone?) & it just makes it all the more sad that she does end up corrupted despite after all the hell it took for her to obtain the Spark of Prometheus. While the subplot involving the Storybrookers trying to infiltrate the castle to get Excalibur wasn't as interesting (despite a few funny moments with Zelena's sarcasm) & Arthur continuing to act like a mad-man fully obsessed with glory, at least it was still something important to the plot. And I'm guessing Belle was on her quest with Merida, or whatever, during all of this because nobody had even acknowledged her disappearance! And with the ending of Emma actually putting Excalibur back together, I really can't wait to see more of what's going to happen now that she has all the power the past Dark Ones had failed to obtain.
I don't think I need to say anything else other than I absolutely loved this episode from start to finish! With solid acting, very compelling & heartbreaking flashbacks & high emotional stakes, all I can say is go & watch it now!

Hmm, curious how she'lll be able to put the dagger & Excalibur back together...

Wow! Didn't think  Merlin would be living for over than a millennium...

Interesting origins of the Holy Grail!

OK, didn't see that coming!

Well, that was a good explanation of his magical powers!

Yeah, I get that you want to save Emma, but maybe you should listen to Merlin!

Wait, how can that be 200 years earlier when Rumple was about 300 years old when he became the Dark One & Merlin had fought the first Dark One for centuries?! I know, they'll be fixing this on the DVD to 500 years, but you'd think the writers would have a grasp of their own timeline that's all over the place!

Aww, the Apprentice looks cute as a kid!

Very mysterious woman if she wants revenge on Vortigan.

Aww! 

So they'll need the Spark of Prometheus to make Excalibur  whole & to communicate with the first Dark One...I have an inkling over who it is, but I'll give them the benefit of the doubt! And given that Emma is corrupted in Storybrooke, it's pretty clear now that Merlin died (NO!!!)

Really don't care...moving on!

Aww, can't help but find them sweet!

So the freaking Holy Grail was in Nimue's living room the whole time?! Not sure she should use it to become immortal...after all! All magic comes with a price!

Pretty creepy...

Yeah, I think Zelena's face sums up my thoughts on some of the stupid decisions the writers have made...

Bit weird she's actually helping them! Unless she has her own hidden agenda...

Well, at least they're smart enough to do their end of the bargain after they get the sword! And very nice of Mary Margaret to guard her and exchange pregnancy tips!

Really can't take Arthur seriously now if the only way he can be seen as a threat is using any magic & acting like a mad-man!

At least Emma's being honest about the darkness slowly winning her over!

Can't help but find Nimue's obsession with revenge a bit scary now...I get why, but still!

Hmm, seems Vortigan has already tried beating them to it!

OK...why is she lying to him?!

HAHA! '*sighs* Oh no.' 'I'm finally trying to do something and nobody believes me!' '(deadpan) It's the hormones talking'

HAHA! 'Zelena...? Are you in pain?' *Zelena knocks her out* 'Well now that you've finally SHUT UP! There's no rest for the wicked.'

Very interesting origins of Excalibur!

Oh crap...

NO!

So the first Dark One still has the Spark of Prometheus in the form of an ember...

I KNEW IT!

Well, she looks...scary!

OK, pretty clear she's not gonna spare Vortigan now even if he destroyed her village & family...

NO!

No, Emma don't listen to her! PLEASE DON'T KILL THAT HOT MERLIN!

''Kill him! Don't go back to being nothing!' 'I'M NOT NOTHING! I WAS NEVER NOTHING! THE POWER YOU HAVE I DON'T NEED! Now, I am going to take that ember from you and you are going to let me.' BOOM!

'How did it feel? To take the right path?' 'To be honest, Merlin. It feels damn good.' YES!!!

About time we learnt the origins of the dagger!

No, Emma...Rumple wanted to use the Dark One powers for good at the start, but look where it got him!

Of course Zelena was double crossing them!

For God's sake Arthur, Merlin may have been cryptic over his prophecies, but you really should listen to him! It's worth more than this bloody hassle over Excalibur!

OK...THAT is scary!

No, Emma...

OK...how is gonna gonna snuff out the light now?