After finding and drinking from the Holy Grail many centuries ago, Merlin gains immortality and magical abilities. He eventually enlists an apprentice, and teaches him. Merlin meets Nimue, who has fled from her village after it was terrorized by Vortigan in his search for the Holy Grail. Merlin and Nimue fall in love, but Merlin seeks to remove his immortality as he does not wish to outlive Nimue. Merlin and Nimue travel to an ancient fire where they reforge the Grail into Excalibur. However, unknown to Merlin, Nimue had drank from the Grail earlier and gained immortality and magic as well. After Nimue rips Vortigan's heart out and kills him, the Darkness takes root in her and she becomes the first Dark One, before breaking Excalibur in an attempt to destroy it. Merlin takes the broken tip of Excalibur and tethers the Dark One to it, making it the Dark One's Dagger. Merlin and Nimue have one last confrontation, where Merlin attempts to kill Nimue with the dagger, but finds that he cannot since he still loves her. Nimue takes advantage of this however, and uses one of Merlin's tears to trap him in the form of a tree. Despite this, Merlin is still able to communicate prophecies to his Apprentice, Arthur and Emma Swan.
When Emma Swan becomes the new Dark One, the Apprentice tells her family and friends to locate Merlin.
Emma, along with her family and friends, meet King Arthur and his knights, who reveal that Merlin is supposedly trapped inside a tree in Camelot. Using a tear of lost love from their son Henry, Emma and Regina manage to free Merlin from his tree form. Merlin frees Lancelot and Merida from Arthur's dungeon. However, when Hook is eventually fatally injured by Excalibur, Emma uses the Darkness within Merlin to tether Killian to Excalibur, therefore keeping Killian alive, but at the cost of turning him into the first of a new branch of Dark Ones. Hook, seeking revenge against Rumplestiltskin once more, murders Merlin to cast a Dark Curse. Merlin's corpse is later destroyed by Emma. In Storybrooke, they reveal a message from Merlin using the Crimson Crown.
- Nimue (ex-girlfriend)
- He is based on Merlin from Arthurian Legend and The Sword in the Stone.
- Additionally, he alludes to the sorcerer from The Sorcerer's Apprentice.
- S4, E19: "Lily" (flashback)
- S5, E01: "The Dark Swan" (flashback)
- S5, E02: "The Price" (flashback)
- S5, E04: "The Broken Kingdom" (flashback)
- S5, E05: "Dreamcatcher" (flashback)
- S5, E06: "The Bear and the Bow" (flashback)
- S5, E07: "Nimue" (flashback)
- S5, E08: "Birth" (flashback)
- S5, E10: "Broken Heart" (flashback)