Mjarin made his name as a powerful mage and the personal adviser to Emperor Leitrim, the ruler of the Cath Bruig during the Wind Age. Helping to shape the policies of the Empire and guide the decisions of its leader, he held considerable sway over the courts of Llancarfan throughout the third era. It is from this position that he bore witness to the rise of Connacht the Wolf, a fierce and capable warrior from the clans of Gower who had accomplished what no man before him had achieved: victory against the Myrkridia. The Myrkridian scourge had held mankind in check since the dawn of the Wind Age, and as Connacht's reputation spread, it was not long before he would be called before the Emperor, who was eager to find a means to end the tireless assault of the Myrkridia. In spite of Mjarin's adamant protestations that Connacht could not be trusted, Leitrim put him in command of a sizable contingent of his own troops. Over several months Connacht transformed the men at his disposal into a fearsome band of elite soldiers, and with the Emperor's support would go on to not only eradicate the threat of the Myrkridia, but also that of the Trow. In time, he would even face off against Moagim the Faceless Terror, thought at the time to be The Leveler, defeating him as well.
As the war came to a close and Moagim lay dead, Mjarin revealed his true intent: to ascend to the throne of the Cath Bruig. Connacht instinctively realized as the mage confronted him that it had been Mjarin, with his powerful dream magics, who had resurrected the devil Moagam and set him loose upon Llancarfan. Despite his unassuming visage, Mjarin wore the mantle of The Leveler, and for the first time Connacht knew the true reason for Mjarin's antipathy toward him. As The Leveler's avatar, Mjarin's sole duty lay in creating a cloud of chaos and destruction that would engulf the entire world, with himself at the center. Connacht was his main impediment to this end, and thus he beset him as he returned to Llancarfan, aiming to destroy any who would oppose his ascension in one deft maneuver. Unfortunately, Mjarin underestimated the strength of the barbarian lord. Even battle-weary as he was, Connacht was able to fight his way to Mjarin and decapitate him personally. Though Mjarin was essentially rendered powerless, his head through some magical means retained its function, as it continued to spout its hatred even after being severed form its body. The war was won, though what became of Mjarin's head thereafter remains a mystery...
After more than a millennium's absence, Mjarin's head (known in the Sword Age as The Head) would resurface during the Great War to wreak further havoc. It is not known how The Nine, the sorcerer-generals of the Light, first came to be aware of The Head's location, only that it was discovered during the summer of the seventeenth year of the war buried under several tons of sand and rock. Claiming to have been one of the Avatara of Connacht and an enemy of Balor, The Head quickly gains the Legion's trust by helping them turn back the armies of the Dark during the seige of Madrigal. Unaware of The Head's history of malevolence, The Nine are quick to follow his advice after he provides them with the means to defeat Shiver, one of the Fallen Lords. Encouraged by their recent victory afforded them by their new ally, the Legion begins to follow other leads suggested by The Head, turning next toward the ruins of Covenant in order to retrieve the Total Codex, a tome of infinite knowledge.
It would be King Alric, leader of the Avatara, who would first come to realize the duplicity of The Head. After being sent by Mjarin over the Cloudspine in search of a mystical suit of armor, Alric is captured and held prisoner there by The Deceiver. Word of the Avatara's imprisonment eventually reaches The Nine, who begin to wonder about the circumstances surrounding it, specifically questioning why he was sent into dangerous territory held by the Fallen. When Alric is eventually freed, he is adamant that he had been double-crossed, and that The Head is not who he claims to be. In spite of mounting evidence to support accusations of The Head's treachery, Mjarin is able to gather a sizable number of supporters to his own defense. Contention over The Head eventually devolves into civil war, with many lives lost in the conflict, including a number of The Nine. Having left the Legion's leadership in utter disarray, Mjarin's head once again vanishes without a trace.
In life, Mjarin was one of the most potent spellcasters of his time, able, among other things, to summon and bind other powerful beings such as Moagim. So great was his power that the Avatara Myrdred, friend and fomer ally of Connacht, was willing to pledge his services to him as an apprentice despite knowing that he was The Leveler. He was a master of dream magic, thought to be the most powerful form of spellcasting in the world of Myth, and it was likely this knowledge specifically that Myrdred coveted. Mjarin was also a keen manipulator, able to coax those around him into carrying out his will often without even realizing it. Even though he lost his mastery over the arcane after his decapitation by Connacht, his ability to subtly exert his influence over those around him seems to be no less acute for lack of a body, as evidenced by the extensive damage caused by his deception during the Great War.