Even Rio knew how dangerous this power was.
Using this power, Rio can literally create new 'Souls' all by himself.
He would just need to extract pure soul energy from others, and then he can create a new 'soul' from that soul energy in whatever form he wants to.
Then Rio can create an 'Artificial Sea of Consciousness' inside that soul, and he can give any kind of memory to that sea of consciousness.
He can literally create 'brainwashed' creatures by just adding memories of being loyal to him in the artificial souls and giving that soul to him.
Not only that, but Rio can actually transplant a soul fragment to another's soul and make them loyal to him in a matter of milliseconds.
'This is a signature power of a final villain-type character…'
Of course, Rio is still not proficient in this and still needs more improvement, but he can surely reach this goal soon.
