For as long as anyone could remember, Tennou Myoga led Oniga to prosperity, and was well-known for not only his kindness, but his shining relations with the Emperor of Sigrogana, Elial S. Erachial. This persisted until out of nowhere, and for no discernable reason, the Tennou suddenly declared war on Sigrogana, initiating the Oniga-Sigrogana War.
Ever since the day he declared war, the Tennou seemed like a different person, hellbent on taking the continent of Sigrogana, even if it meant draining Oniga dry for the sake of war. Even a personal visit from the Immortal King could not sway his newfound convinctions. Over time, the Tennou's demands pushed Oniga's resources to the brink, sparking something of a civil war amongst the Onigans.
His reign, and the war, ended when, despite his Kusanagi bodyguard, he was assassinated. It is commonly believed that Myoga was assassinated by a mage, which led to the ban on most Arcana (and the restrictions placed on the resident Mage Guild) in Oniga. His position was succeeded by his son, Ang Azanbe.
Despite the decade that passed since Myoga's death, most have little idea of how the former Tennou would go from a kindly ruler to a warlord that went after his best friend's land. The truth, known only to an enlightened few, is that Myoga had long held the power of the late god, Zera. This power grew over time, until it eventually overwhelmed the Tennou, imprinting him with Zera's warring nature.
Since Myoga obtained this godly power before the birth of his children, it is entirely possible that the power of Zera was passed down to his son and/or daughter.