Try

'So it was like that…' Vale nodded solemnly as he finally realized what the side effect of overextending Phantasm State was!

Denise had mentioned to him about this side effect before and he also read this in the book.

However, it wasn't mentioned specifically what side effect it was. He only knows that it's certainly not good for the Practitioner of the Darkness Arcane Path.

He didn't expect that the side effects would be too severe if he ignored the limits of his body. 

'Internal injuries can still be cured by the current level of Alchemy according to the books I've read. However, being possessed by an Evil Spirit isn't something a mere student like us can deal with.' Vale thought as he couldn't help but recall the Evanescent Vessels of the Dark Arts Faction.