The Scarlet King is an eldritch god formed as the manifestation of the tension between the modern world and the pre-modern world and the rage it creates. Originally it existing as the concept of there being something greater than man, such as gods or the supernatural, or magic. As modernity approached and people began to understand their world through science and technology, the tension between both worlds raged and led to it's creation.
As magic has disappeared from the world, the Scarlett king must enter our plane through normal means. This god was being into the mortal world through one of the king's brides as completely human. However, his mitochondria DNA came from this demon. Mitochondria DNA is separate from the DNA of the nucleus, which comes from it's human parents and takes up most of the cell. It is donated by the mother, although has genes separate from her.
What I need is an explanation for the mitochondria to take control of its cells nucleus in the body on a full scale. This will slowly transform the individual into the demon it was meant to be at a certain point. Through what means can mitochondria be used to hijack the cell to make this happen?