First Appearance: Strange Tales #134 (July 1965).
Appearance: Many others.
Years Active: 6th Century C.E.
Merlin was, of course, the sorcerer of Camelot. In the Marvel universe his origin (as in the Arthurian legends) are undefined, although they essentially are the same as in the myths themselves. It is known he was active during the Arthurian years, helping Arthur fight against evil; during this time Merlin encountered a number of 20th century heroes, including the Black Knight and Iron Man, and fought against a number of villains who would later fight against 20th century heroes, including Morgan le Fey and the Dragon of the Moon.
Donald Campbell contributes the following:
A possible origin for Merlin appeared in Namor #62. In the story, an injured Prince Namor loses consciousness and has a dream/vision in which the spirit of Kamuu, the last monarch of antedeluvian Atlantis, appears to warn him that the reincarnated sorceress Morgan Le Fey is planning to restore her sunken kingdom of Avalon, threatening Namor's undersea Atlantis . In the vision, Kamuu showed Namor that not all of the continent of Atlantis sank beneath the ocean in the Great Deluge and that some land remained above sea level as islands. Two of these islands were Avalon, the largest of the group, and Ruta, where the Sun Temple was built atop a volcano.
According to Kamuu, Igraine was a sea-princess of Avalon who had married a man of the Sacred Clan and born him a daughter, Morgan. Myraddin was a young Priest of the Sun who first noticed that the flame that was the symbol of their faith was leaping, a sign that a catastrophe that had been foretold long ago was imminent. The ancient Priest of the Sun was too feeble to make the long journey to safety so he entrusted Myraddin with the task of saving their sacred symbols and the old priest's daughter and granddaughter, Igraine and Morgan. The threesome managed to buy passage ona merchant ship from Tingail and leave Ruta not long before the volcano erupted, generating three great tidal waves that swept in and swallowed the islands forever.
After arriving safely in Britain, Myraddin changed his name to Merlin and established his druid worship at Stonehenge. Merlin arranged for Igraine's marriage to Gorlois, duke of the Tingail islands, who adopted Morgan. Sometime later, Uther Pendragon killed Gorlois and took Igraine as his queen, who later gave birth to Uther's son, Arthur, who grew up to become the legendary King Arthur of Camelot. Morgan followed Merlin to Aran Island and became his apprentice after promising, falsely, to become his lover in return. After years of thwarting Morgan's many attempts to bring ruin to Camelot, Merlin finally imprisoned her in Castle Le Fey.
As I said earlier, I consider this a possible origin for Merlin because it is so unlike the myths about Merlin's origin that may already be considered a "factual" part of the history of the Marvel Universe. There are two main myths about Merlin's life before Arthur. The first myth claims that Merlin is the son of a virgin princess from Wales and the demon sent by Satan to seduce her in order to create a superhuman, half-demonic/half-human being to menace mankind. The second myth deals with King Vortigern and the tower that kept falling down. Vortigern was told that he would only succeed in building the tower if he sacrificed a "boy with no human father" and so his men found Merlin in a town in Wales and brought him to the king, whereupon Merlin used his powers to reveal that the tower was really sinking because there was a pool inhabited by two dragons beneath the foundations, a statement which proved to be true. If either or both of these myths are considered to be true in the Marvel Universe, then it would seem that the origin presented in Namor #62 must be false. On the other hand, since those myths about Merlin's origin and youth have never been established as being accurate, then maybe the origin revealed by Kamuu's spirit really IS true and the myths about Merlin being the son of a demon are just myths, spread by Arthur's enemies to discredit the king (or even by Merlin himself to enhance his reputation). I just can't say for sure that it's "true" but I like this new Myraddin origin and I hope it's "authenticated" someday.
As for Merlin's fate after the fall of Camelot, I refer you to Iron Man #150 (in which King Arthur reveals that Merlin has succumbed to his pre-destined fate of being imprisoned in suspended animation by the sorceress Nimue); to the Black Knight mini-series (in which it was revealed that after the first Black Knight was mortally wounded by Mordred, Merlin's astral form appeared and cast a spell upon him that would enable his spirit to return to Earth whenever Mordred's spirit returned to do evil); and to Iron Man #250 (set in an alternate future Earth of 2093 AD, in which Merlin is shown to have awakened from his enchanted sleep in order to help the reincarnated King Arthur deal with a menace that threatens Britain and the world). If these stories are considered to be true, then Merlin is currently still trapped in suspended animation in an enchanted cave somewhere.