Mirus is from the planet Dathomir, born to a small, family-based clan of witches. He is the son of a Priestess named Lauda, a slave named Sanies who raised him and taught him the basics of combat and twin to his sister Mira. He is also the grandson of the clan's Superior Mother, named Petra, who was a Sith Mistress for quite some time, living for thousands of years thanks to her blending of her native Dathomiri magics and the Sith arts.
It was the latter that Mirus took more of an interest in as he grew, eschewing the native Witch magic and preferring the darker Sith arts that his grandmother studied despite her hatred of the twins. He used that hatred as a test; surviving the wrath of a Sith Mistress was not a particularly easy thing to do. Instead, he used his time at the clan stronghold, built into the side of a mountain, to begin honing himself for his eventual departure of the planet. She would not teach him the Sith arts and his mother insisted that he learn Dathomiri magic.
At the age of twenty-five Mirus left his home world of Dathomir to explore the Galaxy and learn more about the Dark Side of the Force. While his mother complained, his father in his own quiet way supported his son and helped him get off the planet. This allowed him two years to begin roaming and learning, studying the Galaxy as much as he could (beyond his already considerable studies) and eventually coming across the Dark Jedi Brotherhood and its Sith Order. Very gladly he submitted himself to their teachings, hoping to learn as much as he possibly could about their ways.