Merlin

Merlin is a minor supporting character on Once Upon a Time. A very powerful Sorcerer who, many years ago, did battle against a being known simply as the Darkness, Merlin became trapped as a tree due to a moment of weakness, and waited years for his prophecies to pan out so that he would one day be free again. During this time, he predicted that Arthur would become King of the broken kingdom of Camelot, and that one day Emma Swan would pull the sword from its stone, with dire consequences. He acted mainly through his Apprentice, who interacted with and performed spells for people in need on Merlin's behalf, while simultaneously guarding his hat.

Before the Curse
Merlin is wandering through the nighttime forests of Camelot with the Dark One's dagger in tow, while the Dark One itself travels in swift, fleeting movements all around the powerful Sorcerer. Using the dagger, the latter is able to command the evil magical being to stop, and it stands to face him - a cloaked figure swathed all in black save for the golden mask that adorns its face. It stands there, staring at him, and Merlin is in tears; he tells the darkness that it destroyed the only woman he ever loved... and so now he shall destroy it. He raises the dagger, ready to kill the Dark One, but he hesitates. It continues to stare at him while he continues to weep. Finally, he utters that he can't do it, proceeding to drop to the floor. The Dark One sees that its dagger has been dropped also and promptly picks it up, then holding Merlin's head up by the hood as he professes that he misses his lost love and collecting one of his heartbroken tears with the blade. It glows purple for a moment, and the Dark One is then able to wield this power to turn Merlin into a tree. He stays in this state for possibly thousands of years.

A young peasant boy by the name of Arthur claims to his friend Guinevere that the great Sorcerer Merlin is trapped within a large, magical tree in their village, and that said tree told him a prophecy that, one day, he would be the one to relieve Excalibur from the stone, therefore becoming King of the broken kingdom of Camelot. He is bullied for these predictions, but ultimately Merlin's prophecy comes to pass, and Arthur is crowned. However, to his horror, when he does finally draw Excalibur, its tip is missing - its other half is the Dark One's dagger, and Arthur works tirelessly to find it, all the while lying to his people and becoming so strung-out that Guinevere is almost driven into the arms of another man. The King continues to transcribe Merlin's prophecies, giving him answers such as how to enter the Vault of the Dark One, but all he's able to obtain is some magic sand that makes his broken kingdom - and marriage - appear fixed.

The Apprentice rushes into the Sorcerer's lair in order to warn him of the fact that the Author forsook his holy duty by using his magic to manipulate events majorly, having made it so that Maleficent's child had the savior's darkness siphoned off into her before being banished to a land without magic. The Apprentice warns the Sorcerer not to worry, however, for he trapped the Author inside the book - from inside it, he may only be able to record events; manipulation is impossible. He is curious as to what will become of Snow's and Maleficent's respective daughters, and the disembodied Sorcerer says that their fates shall remain entwined, just as they always were, telling his Apprentice to just make sure that the Author is never allowed to manipulate fate ever again. ("Lily")

Early Life
A six-year-old Emma Swan heads into a movie theater in Minneapolis to view a screening of The Sword in the Stone, however, because she can't afford her own snacks, she ends up stealing an Apollo bar from some woman's coat pocket. When she takes a seat and begins to unwrap it, Merlin, disguised as an usher, tells her, "Don't." She wonders what he means and he repeats himself, going on to say that, one day, she's going to have the opportunity to pull Excalibur from the stone, just like the young man Arthur is doing in the movie. However, she shouldn't, because if she does then bad things are sure to happen. She heads this warning before Merlin is shushed by some little girl in front of them, and then he disappears suddenly, leaving Emma mightily confused as she watches the cartoon boy pull the sword free.

Before the Third Curse
When Emma and the others are welcomed into Camelot by King Arthur and the Knights of the Round Table, they are told that, due to the fact that Merlin prophesied their arrival, enough time was had to organize a royal ball in their honor, which they all look forward to. They proceed to wonder when the quest to find Merlin will begin, but Arthur reveals that no questing will be involved - he already knows exactly where Merlin is. It is then that Emma, Regina, Robin, David, Mary Margaret and Hook are taken outside and told of Merlin's fate. The latter is shocked that the most powerful sorcerer in all the realms is trapped inside a tree, and Arthur assures the pirate that he expressed the same exact sentiment the first time he saw it too, but all hope is not lost, for Merlin also prophesied that one of them is the savior who will be able to free him. This regards Emma, but Regina pretends to be the savior so that Emma, being the new Dark One, won't be forced to give in to her dark impulses by using dark magic. Regina and Robin later peruse the tree when Sir Percival approaches the former to give her a necklace, which turns out to be enchanted to spy on her. He learns that she's really the Evil Queen and makes an attempt on her life, only to end up dead himself.

Emma enchants a dreamcatcher in order to see how Merlin became a tree, learning that the Dark One used the Sorcerer's tear, which was shed due to heartbreak. As such, Regina deduces that a similarly-shed tear will be the key ingredient in freeing him, and so the dreamcatcher is used to make her relive the loss of her first love Daniel. However, since her heartbreak isn't fresh enough, with her having moved on with Robin Hood, the spell doesn't work, and they have to use Henry's tears instead when his first crush Violet tells him that she's only interested in being his friend (in fact, Emma manipulated the situation by ripping out the girl's heart and forcing her to say these things). Arthur and his knights charge at them as they're about to do the spell, to little effect, and it isn't long before Emma drops in the tears and wields masses of both light and dark magic. She converges it on the tree and, finally, Merlin is freed. He greets Emma, having been expecting her, while he cites Arthur as a disappointment, to his anger. The Sorcerer is then taken to Granny's Diner where he unfreezes Emma's parents, using his magic to take off the enchantment that Arthur had Guinevere put on them. They wonder if he's able to remove the darkness from Emma, and he says he can... provided Emma's heart is ready to let it go, which she seems unsure of.

The door to Arthur's dungeon in Camelot bursts open, and Charming and Hook come on through, easily besting the guards on the other side who have already been weakened by Merlin's magic. The Sorcerer himself is there too, followed by Belle, and easily knocks out more guards on the way, commenting that wielding magic is just like riding a bike. Charming wonders how he knows what a bike is, and Merlin asks the prince if he thought all of his prophecies came true due to luck, then beckoning each of his new allies to hide while a group of guards goes past. Hook realizes that the Sorcerer really can see the future, and the small group go on to find Lancelot's cell. At first, the former Knight of the Round Table is worried by the prince, but he assures him that he and Snow are now free of Arthur's control. Lancelot wonders how they got free, at which Merlin reveals himself; Lancelot is humbled, and Merlin proceeds to have trouble getting the bars off his cell - as well as Merida's, which is opposite. Belle shows him a passage from her book that she believes might be helpful, and it is; the Sorcerer cites her as clever before successfully freeing the prisoners. Later, Charming and Lancelot can be seen in Granny's Diner while Merlin reminds Emma that they once met when she was a child. He warns her once again not to pull Excalibur from its stone when the time comes.

Season 5
Regina reveals that Emma was successful in freeing Merlin in Camelot, to the surprise of the others. The question is raised of why he didn't remove the darkness from her, and Regina proposes that they find out by communicating with him, having figured out a spell that will allow them to do so using the Crimson Crown. Belle is upset that everyone seems less concerned with rescuing the captive Mr. Gold and storms out, proceeding to be chased by Merida and her arrows in the Dark Swan's attempt to make her predecessor brave. Meanwhile, a cauldron is prepared down in Regina's vault, and all that's needed to reach the Sorcerer is for one of his "chosen" to drop the Crimson Crown into it; they summon Arthur, who suggests that he do it alone since no one else has ever been around when he received Merlin's prophecies in the past. Not wanting to take any risks, they all give him his privacy... only for him to throw the magical toadstool onto the fire beneath the cauldron, clearly not wanting the heroes to contact Merlin. This is deduced when the reasonably Crimson Crown is found merely charred - for magical toadstools don't burn - and Regina realizes that there's one other person in town who was chosen by Merlin: the Author. As such, they have Henry drop the toadstool into the cauldron and the message from Merlin appears, suggesting to them that Emma did something awful to him during the missing six weeks; they are told that their best hope lies with Nimue.

Gallery
Gallery of photographic stills released to promote the character.