Before there was Dumbledore... before there was Elminster... before there was Gandalf... there was Merlin. The originalbearded old wizard (unless you're counting Odin), this Public Domain Character was added to the Arthurian mythos around 1100 AD. Merlin was the trusted advisor and surrogate parental figure to King Arthur.
In one legend, Merlin was intended to be an Antichrist figure, begotten on a virgin by the Devil. His mother, however, had the boy baptized at birth, freeing him from the Devil's influence. His demonic heritage gave him the ability to see into the past and future, a gift that is often carried over to myths that do not include this origin story. In other myths, his father is an angel, a fae, or even no one. He is also frequently depicted as a shapeshifter and with a trickster's personality.
In most legends, Merlin's magic helped Uther Pendragon seduce and bed another man's wife (Igraine), leading to the conception of Arthur Pendragon, who he prophesized would be a great king. Merlin was given care of the boy, whom he raised as his son and prepared for kingship. In some myths, Merlin created the magical sword Excalibur and the Round Table of Knights.
Merlin's final fate varies from telling to telling. In some recountings, he loses his wits when Arthur is slain. In others, he is tricked and imprisoned by the witch Niviane (or Nimue), whom he deeply loved and taught magic.
For the 2008 British TV series, see Merlin. For the 1998 miniseries, see Merlin.
Tropes usually displayed by Merlin:
Adaptation Displacement: Merlin is based on the now almost unknown Welsh figure Myrddin, who has his own set of legends.
Anti Anti Christ: Merlin's mother was raped/seduced by either the Devil or some random demon, but she had Merlin baptized as soon as he was born. This relieved him of his Antichrist status but still let him keep his nifty magical powers. Since Medieval Christianity generally thought any sort of power must come from either God or Satan, this story explained how Merlin performed his magical acts without being a saint.
Bowdlerise: As Christianity gained more influence, Merlin became less of a trickster and mage and more of a wise old mentor. His demonic parentage, however, became a part of the story because of Christian influence. The point was to explain why a heroic character was able to use godless magic. His moral ambiguity was likewise explained as the goodness of his mother and evil of his father struggling in him.
Canon Foreigner: Merlin did not appear in the original myths, although he's been around longer than Lancelot and the Holy Grail.
The Chessmaster: Oh so much. He's behind so much of what happens in Arthurian legend.
The Chooser of The One: Merlin is basically behind the creation of King Arthur and why Arthur becomes king. Not to mention he's basically behind many of the factors and events in Arthurian legend, such as the creation of the Knights at the Round Table, Excalibur, the Sword in the Stone, etc.
Half-Human Hybrid: Merlin is traditionally depicted as the son of a woman (sometimes a witch, occasionally a nun) and an incubus. Or, sometimes, a man and a succubus. This is often given as an explanation for his magical and prophetic abilities. Modern interpretations of the legends vary significantly on Merlin's parentage.
This was the result of the Christianization of the legend, to explain how Merlin could wield magic powers (which are always Satanic), but still be a good guy. The woman incidentally is nearly always a raped nun who dunks her newborn into holy water to wash evil away from him as soon as he is born, but he still grows up a horny bastard with a taste for young virgins—the modern tellings tend to forget that aspect of his character.
Pretty sure in both the Christian and non-Christianized versions of the Arthurian Tradition Merlin was depicted as something of a fey spirit. So, half fairy was more like it. See works like the Elfin Knight, which predates most of the Malory as we know today. In the History of the Kings of Britain, Merlin was depicted as born from a rather consensual experience. Try not to think about that too much.
And if you want to go back to the source material with the myth of Myrddin and his sister, it's implied that they both have 'magical' heritage. However, the emphasis is more on Myrddin's far-reaching Sight than anything else.
The Man Behind the Man: In regards to Arthur. According to legend, many thought that Arthur was Merlin's puppet and that Merlin was the one who was actually in control of Arthur and his reign as king. Which might be true, considering how much of a strong influence Merlin is in Arthurian legend and all the events that happened.
Several characters in the Marvel Universe have claimed, with varying levels of conviction, to be the Arthurian Merlin. The first was an illusion-using Silver Age villain subsequently retconned as an imposter, but the usual one is a powerful, manipulative, guardian of reality itself who sometimes uses the name "Merlyn" with a y, and probably was the Arthurian character.
The 1980s Doctor Who Magazine comics featured Merlin as one of a group of "high evolutionaries", guardians of the universe who also included the Time Lords' founder Rassilon. The concept and characterisation were very similar to the Marvel example in this section (not to mention the use of the term "High Evolutionary", which is a Marvel character), suggesting that this was one of several subtle attempts in this era of the strip to link it to the Marvel Universe.
Merlin has appeared from time to time in DC Comics, usually in connection with heroes linked to the Arthurian myths. In particular, he plays a key role in the origin story of The Demon and several of the lead characters in Demon Knights.
Merlin is one of the most widely used recurring extra characters in the Belgian comic book series De Rode Ridder.
The Doctor of Doctor Whois Merlin. Possibly. In the future of an Alternate Universe. It's... complicated. Whether there's any connection between the Merlin incarnation and the Doctor Who Magazine Merlin has not been mentioned.
A less-than-competent Merlin appears in the French comedy series Kaamelott.
he is in fact a Celtic druid rather than the usual sorcerer.
The short-lived 1980's sitcom Mr. Merlin had him still alive in the present day, keeping a low profile under the name Max Merlin. His costar is a young guy who learns Max's secret and then gets taken on as his apprentice.
Old Welsh tales and triads tell of a wild man of the woods living with pigs beneath apple trees, who had been a prophet and maybe a shaman, Myrrdin, or Lailoken (northern reaches, a Scottish variant), who lived in Coed Celiddon, the Caledonian Woods. After he had published his grand history, Geoffrey of Monmouth apparently found some older poetry or sources and published the Vita Merlini, Life of Merlin, and a very different Trickster from the king-maker of Uthyr and Arthur was he. Taliesin was a rough contemporary, a famous poet later given some magical or prophetic prowess as well.
Geoffrey of Monmouth's History of the Kings of Britain (ca 1135) introduces Merlin as the child of woman but with no father, whom Vortigern wanted to sacrifice to keep a castle from crumbling above its foundations. Instead of being killed and having his blood slake the foundations, Merlin spoke prophecy and showed two dragons fighting beneath the castle, a white one (English invaders) and a red one (native Britons or Welsh). Merlin lived, Vortigern did not, and Merlin went on to aid Uthyr PenDragon in seducing Igraine the wife of Gorlois and so father Arthur in Cornwall at Castle Tintagel.
Geoffrey of Monmouth also wrote a Life of Merlin, reflecting the old Welsh tales of Myrrdin as wild man of the woods. Having seen his sister's husband slain in battle, Myrrdin flees to the woods to live in or beneath an apple tree as a vegetarian hermit with a pig for a companion.
Robert de Boron's book on Merlin introduced the antichrist element.
Thomas Mallory (or Malleore) and his Le Morte Darthur is the primary source for T.H. White's Arthurian works.
Mary Stewart's The Merlin Trilogy, based on Geoffrey of Monmouth, has Merlin telling his own story, as she fills in some gaps left by Geoffrey and by Mallory. A fourth book, "The Wicked Day" follows with Mordred's story.
Mercedes Lackey's Gwenhwyfar borrows this element. It also explains the myth of him sealed in a tree with him having a stroke, which leaves him paralyzed and catatonic (wooden and tree-like, in the words of one character).
Prior to her this novel, Mercedes Lackey's Urban Fantasy novels mention that Merlin was one of the greatest human Bards (magic users) in history.
Knight Life by Peter David follows T. H. White's Merlin in aging backwards to the 1980s or 1990s, finally reaching childhood.
Merlin was apparently a real historical figure in the Harry Potter universe. He gave his name to an order of powerful wizards, the Order of Merlin. His name also gets used in an Oh My Gods! fashion a number of times, usually as "Merlin's beard!" Pottermore, interestingly, reveals that he was placed in Slytherin during his time at Hogwarts (which, when you consider all the political scheming that went on with him in the legends, makes more than a little sense).
Similar to the Harry Potter example, Merlin is a historical personality in The Dresden Files (as is Arthur), a founder of the White Council of wizards, and one time guardian of the holy sword Amoracchius (aka Excalibur). The head of the white council is traditionally given the title of "The Merlin". The current bearer of the title is Arthur Langtry, whom Harry note that he "resembles what you'd think Merlin would look like". And to make clear, Merlin was the Archmage's Archmage. He constructed a prison for Physical Gods and Eldritch Abominations of which Skinwalkers are the least secured of the lot. It was constructed at once during at least five different moments in time to make sure it was protected over time and space. It was said his mentor was Odin, Norse God, Winter King of the wyldfae. And his direct line of students who grow to teach new wizards reaches to Harry Dresden personally.
Sir James Knowles' The Tales of King Arthur and His Knights gives an incredibly abridged version of Merlin's involvement from the beginning, but it all comes out with Merlin seeming much younger than other versions (the BBC series excluded) portray him to be (he's a child - so maybe 7 - when Vortigern uses him. Guestimate 10 years before Arthur is born...means he wouldn't be 60 when Arthur was 15 [yes, 15, not 11, Disney]).
In the fairy tale "Childe Rowland", the "Warlock Merlin" gives invaluable advice to the eponymous hero.
In The Dark Is Rising series, the mentor character Merriman Lyon is revealed to be Merlin.
In The Weathermonger by Peter Dickinson, the mysterious Changes that drive the plot turn out to be a consequence of Merlin being rescued from his imprisonment.
Merlin is a supporting character in the children's fantasy novels Merlin's Mistake and The Testing of Tertius by Robert Newman. His eponymous mistake was an attempt to magically endow his student Tertius with all wizardly knowledge in one go; it went wrong, and Tertius ended up with a head stuffed full of anachronistic twentieth-century science know-how instead.
Another young adult novel, The Magic Cave/The Hidden Cave by Ruth Chew, concerns two young children who find Merlin trapped inside an oak tree (although he's there by his own error, thanks to wanting to knowwhat was inside an acorn, rather than due to Nyneve). The titular cave is actually a transformed drainage pipe which uses portal magic to take Merlin and the kids to various places, such as the library, the botanical gardens (for herbs to do magic), and eventually the museum to obtain an artifact (the Eye of Horus) to take Merlin back to his own time.
The "Nightside" series. Unlike most examples, the author included the bit about him being the devil's son, though he renounced his father. His not-completely-dead body is buried beneath the world's oldest bar, tended by his bastard descendants.
We also get a look at his Bizarro World self, who became the Antichrist, corrupted Arthur, and rules over an apocalyptic Britain.
He made his appearance as Arthur's mentor in the 1960s musical Camelot, which was made into a film in 1967. In this version his name is spelt "Merlyn".
Merlin was a 1983 Broadway musical starring legendary Stage Magician Doug Henning. The story was an original one, depicting his battles with an evil sorceress queen in the days before he met Arthur.
Similarly, he was a character in one segment of the Las Vegas musical/special effects showcase EFX (1995). As he shows Arthur the world's natural magic, they get ambushed by Morgana. She transforms herself into a dragon to defeat them, but Merlin does the same and wins the day.
The Merlin of Mage: The Awakening was an Acanthus archmage, a master of Fate and Time, who engineered the creation of Arthurian Britain in order to Ascend. It promptly got erased from the timeline by the Exarchs, but they couldn't stamp out the mystical resonance of Arthurian symbology.
In Quest For Glory 2. Merlin is mentioned as a member of the Wizard's Institute of Technosorcery (WIT). A magic user can ask him to be a sponsor for your admission to the organization, though he mentions that he is way too busy for that.
Arthur, King of Time and Space has Merlin as different things depending on what era it's set in. In the medieval period, he's the traditional wizard, complete with foiled-Antichrist backstory and prophetic powers (his frustration that nobody ever pays attention to his foretellings until it's too late is a Running Gag). In the space opera period, he's a Higher-Tech SpeciesTime Traveler. In the contemporary period, he's a Cool Old Guy who runs a comic-book shop.
Stupid Mario Brothers has Merlin as a mentor to Mario, but also introduces his evil brother Nox Decious. Along with his brother, this version of Merlin originally appeared in an older project by RMA Studios.
In an episode of ThunderCats, Mumm-Ra disguised himself as King Arthur in order to steal Excalibur and use it to defeat Lion-O. The only reason he was defeated was because he dropped the disguise and started gloating, which prompted Merlin to show up out of nowhere and kick his ass.
Star Swirl the Bearded in My Little Pony: Friendship Is Magic, a famed figure in pony folklore, appears to be the Equestrian equivalent of Merlin. Twilight Sparkle once dressed up as him on Nightmare Night, and at the end of Season Three Celestia gave her the task of completing an unfinished, destiny-determining magic spell found in his journal. Twilight's success in this task, creating new magic in the process, was the catalyst for her transformation into an alicorn princess.
The Visionaries was set on an alien planet, but had multiple allusions to the Arthurian mythos (such as a bad guy named Mortdred). Most notably, the series's central wizard was named Merklynn — although he was decidedly less benevolent than the Merlin of legend.