Kirin, Behemoth, Twintania, Ahriman mount music.

[saber_alter]

Wait, the theme that plays when you fight Lahabrea? Didn't even realize it was the same thing. Hmm, well I wonder if they'll use that for Nightmare as well.

It plays when fighting any ascian, or during turn 5. it would make sense though, kind of gives off a ".VS. big bad" vibe.