Let’s consider it from Itsuki’s per

Let’s consider it from Itsuki’s perspective.
After losing to the Spirit Turtle, Witch’s influence allowed him to unlock the new power known as the Curse Series.
It’s a plot right out of Mecha Anime, and, out of those same Anime, he would have to rest until the power of his Bow settled within him.
Even so, there’s no doubt that Itsuki has been devoured by some form of curse.
Though, that seemed apparent from the start of the incident.