The previous episode of ABC's "Once Upon a Time" took an unusual turn as the show did not feature more of the main characters, but rather on new ones — Merlin (Elliot Knight) and Nimue (Caroline Ford).

During that episode, it was revealed that the episode's titular character was actually the original Dark One. Apart from that, the show delivered a way to make viewers understand how Merlin and the Excalibur came to be.

Merlin and Nimue met and easily made a connection and fell in love. The pre-Arthurian history revealed that Merlin admitted to his true love that the Holy Grail grants immortality to anyone who drinks from it, as long as he or she is worthy. Otherwise, that person would turn to dust.

Since Merlin knows of this, he came up with a different idea, which is to forge a sword from the grail and use it to end immortality, so the two of them can live normal lives.

In a flashback, it was also shown how Nimue's village was destroyed by the Dark One of their time and, of course, she's not as pure as expected since she had it in her heart to plan for revenge if she had powers. The wizard also discovered that someone came for the Holy Grail and then got a vision of the Dark One closing in on them.

The Dark One finally revealed himself and attacked and presumably killed Nimue, leaving Merlin devastated. However, it turns out that she wasn't dead after all since she drank from the grail without Merlin's knowledge. She went after the Dark One, ripping his heart out, which then turned her over to the dark side.

Meanwhile, back in Camelot, Merlin advises the Storybrooke heroes that they can help Emma (Jennifer Morrison) by putting together Excalibur, which was broken in half by Nimue in history. To do this, they need to set out to find the Flame of Prometheus.

As Emma prepared for the journey, she shared a moment with Hook (Colin O'Donoghue), and this moment is believed to be their last for the first half of the season, Entertainment Weekly reported.

In present-day Storybrooke, though, Emma was seen with a group of Dark Ones; the Excalibur is now in its whole form again and Emma takes possession of it.

Emma's dark story will continue to unfold when "Once Upon a Time" returns on Sunday at 8 p.m. EST on ABC.