In my AU, Ghost transformed into the Shadelord, but tried to seal a decent part of his power off, after expending a lot keeping his sibling alive. My AU takes elements of the Godmaster ending and the Dream No More ending, combining my favorite aspects of both.

Ghost expended his power destroying the Radiance, and was fairly normal when he returned and had Quirrel take his shell to be repaired. He went back into his shell once it was repaired, and experienced accelerated growth (He would have been of age already if he hadn't been in the Abyss without any access to the life energy needed to grow), reaching adult stature in under a year. Upon hitting maturity, this form burst out for the first time, leaving his shell behind. At first, he was confused... but this form quickly became far more powerful than he could handle. It caused him to be overtaken by feral instincts, and any time this form takes over, Ghost has terrible trouble keeping it under wraps.