Morgaine Le Fay
Real
Name: Morgaine Le Fay
Voiced by Olivia D'abo
The same Morgaine Le Fay from Arthurian myth, she has survived the centuries and continues to seek out the Philosopher’s Stone, which will grant her the ability to restore Camelot...with her son on the throne.
Real
Name: Mordrid
Voiced by Soren Fulton
The illegitimate child of a union between a King Arthur and his half-sister, Morgaine Le Fay, Mordrid assists his mother in seeking out the Philosopher's Stone, which will allow him to gain control of Camelot.
Commentary
Much like Hades, these are not the original Morgaine Le Fay or Mordrid, rather, these are reimagined characters for the Justice League series (in Le Fay's case, she harkens back to Jack Kirby's original concept of the character from his Silver Age The Demon series). In the original myth Mordrid was an adult who usurped the throne while Arthur was fighting a campaign at Gaul, sowed discord within the kingdom, and finally met his end in a battle with his father in Cornwall. As for La Fey herself, well, it depends on which version of her myth that you read. She was originally considered a benevolent force in early Welsh and Celtic myth, but became steadily more sinister as Christian influence (which didn't care much for powerful female characters, especially those with Pagan influences) seeped into the Arthurian myths and legends.
That aside, it's worth noting that the creative team of A Knight of Shadows took some liberties with Jack Kirby's "original" source material. Originally a lone figure in the DC Universe, they added a youthful Mordrid as a supporting character. This was possibly for the same reason that the Joker had Harley Quinn and Luthor gained Mercy Graves—it's difficult for comic book characters, who are able to use pages of pages of internal exposition to relay their plans to the reader, to translate to television, where they need to give viewers the gist of their plots quickly—but within the context of the episode, however, Mordrid's seemingly eternal youth takes on odd implications.
Consider: why would Le Fay keep Mordrid youthful, when he would be of much more useful to their cause as an adult warrior? I actually came to two possible conclusions to her unusual decision—first, keeping him as a youth would make Mordrid that much easier to control (as a child he would be more likely to rely on her, whereas an adult he might decide to eliminate his mother and rule Camelot alone) and, second, he was kept young in order to keep him open to new trends and new technologies. The cliché of having a child program your VCR for you is all the more apparent in A Knight of Shadows, where La Fey laments in the loss of traditional books, but then is amazed when Mordrid finds the necessary information on the Internet (which he appears to be incredibly proficient in). This may also explain why she needs to magically steal the youth of others in order to live—La Fey's channeling an incredible amount of energy into Mordrid to keep him young, draining her own life energies as a result. This personality trait also parallels her comic book appearances, where she also continually needed to restore her youth.
Images courtesy of The World's Finest and Bird Boy. Additional information courtesy of The Encyclopedia Mythica.