Last Choice
Although Ryu hadn't been surprised by the Quivering Spectral Wings entering his body, he was surprised by The Compendium entering his Spiritual Sea, and that was because he hadn't sensed the aura of a Mental Realm Treasure on it. It seemed to have broken convention and chosen this location for itself based on its own personal preference.
God Treasures definitely had intelligence of their own, so this alone was surprising, but the choice was, especially because Ryu could vaguely feel that The Compendium was influencing his gaze much like his soul did. It seemed to have sensed an opportunity for itself there.
At the same time, much like the Quivering Spectral Wings, it greedily soaked up the energies of Ryu's body, seemingly not being able to get enough. And to The Compendium's surprise, its new owner didn't do anything to stop its actions, even when its pace of absorption increased.