A soul is taken from its rightful rebirth and is sealed for experimentation by another world's God to better understand her ability to alter anything around her. The [Rules] seemingly bent just for her when she willed it, and the Mad God wanted this power for himself.
He however, was killed by his betters for trying to imprison a soul bound to be reincarnated. The soul, after receiving a blessing from that world's Goddess of Fate and Souls, was given a chance at rebirth once more.
However, none of the Gods realized the power of the being they've just released onto their world...