Ravenclaw's figure flashed, turning into starlight that poured directly into the diadem atop Erwin's head. Erwin reached up to touch the diadem on his head—must admit, being a spirit was quite convenient.
With a mental nudge from Erwin, the diadem vanished, or rather, turned invisible.
This was an ability of the diadem that Erwin had learned after acquiring it.
Chapter 171