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As the System foretold, William's body promptly exploded roughly halfway through the upgrade process. The Mana that had been forced out of his body dispersed all of its energy evenly throughout the World, not returning to William until after the Talent upgrade was completed.
Other than the initial scare from the System and wave after wave of Mana fluctuations that shook the World, there really wasn't much damage. Many of the Magical Beasts under William's control were also targeted by the rampant Mana, but it wasn't as damaging to a living body as it was to the environment.
Seeing that his friends and Magical Beasts were safe, William heaved a sigh of relief.
Alright, System. I guess you get a pass this time.
-Don't I always? I clearly answered your questions.
Well... yes. But it's almost as if you want me to die sometimes. William complained.