I do much of my thinking about Gaiden to OSTs from RWBY -- a few of them are
beautifully relevant, probably because RWBY is also a very "loss of innocence / emerging unstoppable enemy" themed kind of thing when it wants to be.
Die,
Time To Say Goodbye, sometimes Red Like Roses II. I've got a mental montage set to Time To Say Goodbye, stuff like babayAnna poking her head out of a barn and seeing her father take an AG bullet before going full valk all over. Later, 16yo Anna rising above an army... Anna growing into Durga one part at a time in a sequence -- as if the armor is swallowing her up, feet then legs then stomach then gauntlets + chest, shoulders and finally helmet, and each scene has her fighting AGs. I had a whole thing planned out at one point, almost considered PMing it to the artists because frankly we could really set something to that OST and it would
make sense... mostly. Couple parts would be weird.