Perhaps the most famous magic user in history, the mysterious wizard known as Merlin has been the subject of countless legends and fictional tales. While many believe he was part-demon, the truth was stranger still. He was in fact an Angel who served the Valar, the heavenly caretakers of Middle-Earth, the Earth that existed thousands of years ago. In those days he was not known as Merlin, but by another name - Saruman the White.
Like his sibling Olórin (Gandalf), Curunír was sent to Middle-Earth in order to help the people in the Third Age to prepare for the coming darkness. A powerful magician and a natural leader, he took on the identity of Saruman and rose to leadership over his fellow Wizards. At the high of his career, he held the coveted seat at the head of the White Council. It could be argued he was one of the most powerful members of the Army of Light.
However, he was not of the light, at least not anymore. At some point pride and greed took hold of him and he sought to aid Sauron and his Army of Darkness. He formed an army of his own, composed of genetically-enhanced Orcs which he called his Uruk-hai. While he had some early success, his armies were decimated by the Army of Light, his fortress of Isengard toppled by the Ents of Fangorn Forest, and he himself was slain by his lackey Grima Wormtongue.
But evil has a way of coming back.
As an angelic being, his spirit drifted through various realms until it settled on a recently deceased mortal whom it could possess. Who this person was is unimportant, what mattered was that Saruman used his powers to sunder what remained of its spirit, take its place within the body, and then raise it from the dead. The miracle earned him some recognition within the ancient community in the British Isles, gaining him the first followers in what would later become the Order of Merlin.
Taking the name of Myrddin, he would use the land of the neolithic Britons as his base of operations, first contesting with his arch-foe The Champion for the title of Guardian of Magic, then manipulating the line of Pendragon to create a great and glorious Camelot Empire. Ultimately his quest for empire was dashed, when he was betrayed by his partner Morgana Le Fey. The war between Morgaine and Arthur ended Camelot and destroyed centuries of planning.
Seeing that great project spiral out of control and burn in front of his eyes, Myrddin was forced to use the last of his magic to send his tower into a pocket dimension which legendarium would label as Avalon. It took centuries to recover from the great spell he cast, requiring him to remain in hiding, rebuilding his armies of Uruk-hai, and crafting grandiose plans for his return.
Yet, each time he attempted a return he was thwarted. When he attempted to aid the forces of Apokolips during its invasion, his own daughter Roma stole the Amulet of Right in order to empower Brian Braddock as Captain Britain. After reclaiming Excalibur, the Sword of Power, from Eternia after the death of its champion He-Man, he was betrayed by his apprentice Sabine Delacroix, who gave the weapon to Kentauride of the Mythic Avengers.
Even after a lifetime of defeat, Merlin persists in his schemes of domination. At a very small gathering of the extremist Norsefire party, Merlin sees a chance at a new empire. There is a brand new frontier ahead for the planet, and S.H.I.E.L.D. and its Special Security Zones are now the keys to his plans.