"Why the Vampire Servant hates the Werewolves"

Grisha and Trevor took over in grilling the meat while Ivory and Sigmund worriedly watched over Zen in his room. They waited for him to wake up. Ivory frequently checked his breathing; fortunately, it seemed like he was not in pain anymore.

But he had rapid eye movements as if he's currently in a dream, so Ivory observed him carefully, and it didn't seem like he's having a nightmare. 

"He's smiling," Sigmund said while observing him as well, "I wonder what he's dreaming of."

Ivory was relieved that he was not having a nightmare, but why did he suddenly have that odd headache and fell asleep? It was as if he was forced to sleep!