• Katherine Langford as Nimue, a young Fey woman who is shunned by her village, as they believe she is "cursed". Her village is soon slaughtered by religious zealots, "the Red Paladins", but she escapes and is entrusted with the legendary Sword of Power by her mother Lenore, who tells her to get it to "Merlin" moments before Lenore's death at the hands of the Paladins. Setting out on a journey to save her people from a holy war of extinction, Nimue struggles with the corrupting power of the Sword, while coming to grips with her secret history, the powerful magical legacy that flows in her blood, and what it will mean for her and her people when she embraces her destiny as the "Wolf-Blood Witch", and eventually as the Fey Queen.
• Devon Terrell as Arthur, a sellsword who becomes Nimue's main love interest and struggles to rise above the shadow of dishonour and debts left in the wake of his father's death. He becomes an ally of the Fey and slowly begins to regain his family's honour on his journey with Nimue.
• Gustaf Skarsgård as Merlin, a once-formidable magician who has lived for hundreds of years and is now the closest advisor to Uther Pendragon. He seeks to redeem a morally bankrupt past by destroying the Sword of Power when he discovers it still exists, revealing it contains dark magic that will corrupt its wielder and was the loss of his formidable magical powers.
• Daniel Sharman as Weeping Monk / Lancelot, the most powerful warrior in the army of the zealots. He is seen consistently to be the turning point in conflicts between human and Fey, and is seemingly able to track and lead the Paladins to Fey wherever they go, killing them without mercy, though he claims to spare children to spread his message.
• Sebastian Armesto as King Uther, a powerful but deluded king who uses Merlin's counsel as a crutch until discovering he can no longer do magic and his larger manipulations. Uther turns to his mother for guidance, but her political aspirations create a rift between them that forces Uther to take his own counsel. He seeks the Sword of Power, which will solidify his illegitimate claim to the Pendragon Throne.
• Lily Newmark as Pym, Nimue's closest friend who escapes the destruction of their village at the start of the series. She is rescued by Dof after being separated from Nimue, and falls in with Red Spear's pirate raiders, masquerading as a healer in order to stay alive.
• Peter Mullan as Father Carden, the leader of the Red Paladins and the fanatic responsible for their formation under the Roman Catholic faith. He spreads the misguided rhetoric of Fey being demons and leads the Paladins in his goals to hunt Fey to inevitable extinction. He manipulates Lancelot into hunting his own kind on the Paladins' behalf as the Weeping Monk.
• Shalom Brune-Franklin as Sister Igraine / Morgana, Arthur's estranged sister who was sent to a convent as a girl but grew to become the greatest human ally of the Fey. She uses her grandmother's name "Igraine" to conceal her true identity at the abbey and shelters Nimue for a time before leading her to a settlement of Fey where the latter soon becomes Fey Queen, taking Morgana as one of her chief advisors. Morgana eventually learns she contains the potential to become Britannia's greatest sorceress from a Dark God.

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In 2018 it was announced that Netflix had given the production a series order for a first season consisting of ten episodes. The show is based on the novel of the same name by Frank Miller and Tom Wheeler (Simon & Schuster 2019). In addition to creating the television adaptation, Miller and Wheeler were also expected to executive produce the series as well. In September 2018 it was announced that Zetna Fuentes is set to direct and serve as an executive producer for the first two episodes of the series. On July 9, 2021, Netflix canceled the series after one season.
In September 2018, Katherine Langford was cast in the series' lead role. In March 2019, it was announced Devon Terrell, Gustaf Skarsgård, Peter Mullan, Lily Newmark, Shalom Brune-Franklin, Daniel Sharman, Sebastian Armesto, Emily Coates and Billy Jenkins had joined the cast. Creator Frank Miller appears in a cameo.
In January 2019, set building began on disused Army land in Deepcut, England. This was expected to continue until March 2019, with filming then scheduled to start and continue until September 2019.

The series was released on Netflix on July 17, 2020.