AmorAstrum
[09] Warrior
What I had thought when Viola says that is this:
Moon = Raphael as Nightmare. If you customize Nightmare's costume his face is clearly Raphael.
Sun = Raphael as a human.
"The Moon will swallow the Sun and it shall never rise again."
Here is my novel of a speculation:
In Nightmare's profile in SCV, it says that he found a body underneath Ostrheinsburg after the events of SCIV. According to Amy's ending in SCIV, there is a possibility that this corpse is Raphael, assuming the fact that Nightmare has his model in-game. Just look at a few minor details -- Graf Dumas' persona is exactly like Raphael; court politician, essentially. Their voices are very similar as well. Soul Edge could be so strongly corrupting Raphael that he does not meddle with things like his identity. Then again, his will for Amy is so strong that his soul broke away from his body -- hence the in-game Raphael with the hat -- and is just merely living as a spirit, still thinking that he is looking for Amy. That could also explain in Raphael's profile "how long had he been climbing that spiral staircase?" and how he just knew that he needed to begin searching for Amy, without questioning that he woke in a cellar. Now, Viola's phrase could simply mean this: Soul Edge is corrupting Raphael so much that he may never reunite with his soul again.
Personally, I think that there is a part in Raphael/Graf Dumas that still wants to search for Amy. After defeating Viola, considering with the theory that she is Amy, Nightmare lets out a moaning sound, as if in pain. Also, one of Nightmare's win quotes is "Now Soul Edge, grant your master his wish!" not really sure about that, but it might be related to Viola.
That's my theory. ._.
Please... marry me? ;w;