( The Ultimate Condition For Core Reformation!
After reaching the ninth stage's threshold of internal energy, get crushed under the butt of a giant beast or something equivalent.
A butt is a butt. It's soft and at the same time, it's hard. It's hot while it's surrounded by darkness. that is what a Butt is. It is the perfect place to reform a core that stays in the darkness while carrying a hot power.
The butt of a giant beast ( or something equivalent ) carries an unfathomable force - Yang energy, which compliments the force dwelling in the darkness - Yin energy, and is the perfect place to sever the core with the outside world.
Staying in this severing natural state for merely half a minute ( if you are lucky, even a few seconds ) would be enough for your core to reform. )