Nyx wasn't sure of the crystal aspect but it sounded as if they were on the track to actually succeed regardless of Nyx's personal knowledge of any prophecies or lack there of in this case.

Prompto surely ha a different point he was making in regards to emphasizing the just the three of us aspect of the waiting but Nyx was most likely picking up on the wrong connotation of the statement.

"It's only the three of you left in the fight?"

Really? What were three people meant to do, or even four if you included the king against the full might of the Empire?