That was certainly a surprise.
Syfy's The Magicians season 5 reveals who the Dark King really is and explains what his plans are in the final season of the TV series.
The Magicians' Review: The Dark King's True Identity Revealed
Black Knight (Sir Percy of Scandia) Powers, Enemies, History
The Magicians Season 5 Episode 3 Review: The Mountain of Ghosts - TV Fanatic
Black Knight AKA Dane Whitman - the Marvel Comics history of Kit Harington's Eternals character
The Magicians Season 5 Episode 3 Review: The Mountain of Ghosts - TV Fanatic
Rupert Chatwin, The Magicians Wiki
The Magicians' Postmortem: Hale Appleman Talks Eliot's 'Turning Point' (and That Sex Scene)
The Magicians Season 5 Episode 3 Review: The Mountain of Ghosts - TV Fanatic
The Magicians' Review: The Dark King's True Identity Revealed
Magicians' co-creator Sera Gamble on Eliot's potential new love interest
Black Clover: Asta, Yuno, and Noelle's current Magic Knight Ranks, Explained
The Magicians Season 5 Episode 4 Review: Magicians Anonymous - TV Fanatic
The Magicians Explains Who The Dark King Really Is: What's His Plan?
The Magicians' Review: The Dark King's True Identity Revealed