The cloaked guy probably isn’t the actual actor. Nimue’s character in the first appearance was played by some bald guy before she was casted. He was in a dark cloak and mask. For all we know, this person is just a stand in for the actual character or stunt double.
(I’m referring to the scene where Merlin was turned into a tree btw. I recall there a bald actor who was casted but we never actually saw him in the show, he just played the cloaked nimue)
"I could have the giant duck as my steed!" --Daniel Radcliffe
Keeper Of Tamara's Taser , Jafar's Staff, Kitsis’s Glasses , Ariel’s Tail, Dopey's Hat , Peter Pan’s Shadow, Outfit, & Pied Cloak,Red Queen's Castle, White Rabbit's Power To World Hop, Zelena's BroomStick, & ALL MAGIC