A Severe Backlash
Of course, this was a total lie. After the merge, the spirit of the master would wither away and only the monster's spirit would dominate the body. This was like living a life inside a prison, a life much worse even than that.
Another hard to meet condition was for the master and monster to have the exact same spirit power. That wasn't limited just to the general grade, but also the total number of spirit power the master and monster had.
Little margin of difference was allowed in the process, and that made it hard for any monster to change into a Mutant. Yet that Nightmare monster seemed to hit the jackpot, meeting a human that got formed out of a speck of dust, one who fulfilled all these conditions.
The interesting scenes didn't stop there. Once the monster changed and became human, a process that William knew happened five years ago, it started to roam the world.