A Mage's TaleCaster's True Name is Merlin. The vagrant of the paradise. The court mage, advisor and prophet that appears in Arthurian legends. He would at times guide, at times torment and at times help King Arthur so that she could overcome many enemies and hardships. Still, even as a wise man he is fundamentally inhuman. For you see, he is a mixed-blood of incubus and human. His mother was a princess of Wales, but his father was supposedly a supernatural incubus that lived between the Earth and the moon.
It has been said that he performed many prophecies at a young age. Among those, the prophecy that caused Merlin’s name to become widely known was one in which, after correctly guessing about the existence of a red and a white dragon sleeping beneath Mt. Emrys, he spoke how those red and white dragons would fight against each other upon waking up. Merlin's words, which were transmitted to posterity as prophecies, indicated the red dragon as Britain and the white dragon as the Saxons and that Britain would gather their forces under a great king and surely reign over Gaul and Rome. He also left behind many other prophecies, among which there were those concerning wars and even the king's death.
He served King Uther Pendragon. Uther loved Igraine, the wife of Duke Cornwall, so he secured Merlin's assistance in winning her love at the price of leaving his child in Merlin's care. Merlin arranged the marriage between Uther and Igraine, watch over King Arthur's since before her birth. Merlin told Uther a prophecy about his child, who would become his appointed successor during their war-torn times. Under an arrangement from Uther, he helped produce an heir bearing the "essence of a dragon", so Artoria was born as a human infused with a dragon's magical energy. Although the child was a girl who could not officially become the successor, Merlin was delighted all the same, believing that her sex never mattered. Confident that her being separated from the castle until the prophecy was fulfilled was proof that she would become king, he entrusted her to the king's vassal, Sir Ector, and had her raised as the child of a knight. Merlin visited at some point and swore Sir Kay to secrecy about the matter of her gender. According to anecdotes, he taught many things to the young Arthur and ascertain her growth as if a foster father-like existence.
He prepared Caliburn for the selection of the next king, but none could pull it out of the stone. As the other participants prepared a jousting match to decide the kingship, Artoria neared the sword and prepared to pull it without hesitation. Merlin appeared before her then, telling her she should think it through and describing that to become "king" is to no longer be "human." Warning her that she should stop because she is fated to die a miserable death while resented by all humanity, he showed her the end she would face. Having prepared for that day since her birth and firmly believing in the people smiling she saw in Merlin's vision, she pulled it without hesitation and became king. As he left her, he said that in exchange for the miracle that she will lose that which is most important to her.
Due to Artoria having issues with discussing her gender and producing an heir, his magecraft was to solve it by making her a pseudo-male for an unknown duration of time, which allowed for the creation of Mordred from her sperm. Artoria lost Caliburn at a later point and obtained Excalibur and Avalon from the Lady of the Lake. It is said in legend that he asked Artoria about which implement was better, chiding her for choosing the sword because the sheath will protect her.
Merlin has left King Arthur before the last battle with Mordred due to his love affairs problem. He claims that the cold-mannered woman has been trying to murder him so he has taken refuge to the Reverse Side of the World where he'd crossed the boundary and stumbled into a foreign land known as Avalon. Merlin had thought that the evil witch could not reach him anymore. But he encountered a gate that was built with rough stones, resembling the stone circles. The gate was inscribed with "Only the innocent may pass". Merlin realized that the gate was a trap and he was imprisoned when he had walked into it. Massive walls of stone emerged from the ground in front of him. The walls seem to vertically extend forever, effectively trapping the visiting magus inside. The place is like a tower without a roof.
Merlin is fated to be trapped in this prison “tower”, called the Garden of Avalon, for eternity, having transcended death. To ascertain the world of humans until the end of the universe, without being able to die. As if to say that this is the fate of a foolish man, who playfully meddled with the fate of a single girl. This also means that there is no possibility of him becoming a Heroic Spirit even if he is worshiped by the masses, since he is unable to die in order to have his soul recorded in the Throne of Heroes.