What lies between shadow and light: Hints and solution

[mmammel]

OK, here is a walkthrough for those who are not inclined to work on the process anymore:
!! The decrypted cipher text follows !!

Spoiler below!

The Fibonacci series with seeds of 5,6 is
5,6,11,17,28,45,73
but add the digits together of each term (28 -> 2+8 -> 10 -> 1+0 -> 1)
OR use mod 9 but translate 0 to 9
Gives us the key:
5,6,2,8,1,9,1….

Translate the cipher text to numbers where A=0, B=1:
YNGPPUZ ->
24,13,6,15,15,20,25

Now SUBTRACT the key from the cipher text, but of course MOD 26 – so if the result is less than 26, add 26 back to it.
The decrypted numbers are:
19,7,4,7,14,11,24
Now just translate the numbers back to letters:
THEHOLY

The final message is:
THE HOLY ETHEREAL SUN AROSE CASTING RAYS EMERGING DAYLIGHT OVER ALL THE HORROR IS SLOWLY BEING RESET OCCULT KNOWLEDGE ETERNALLY NOCTURNAL

So now you know the answer right well, there is one more step – can you see the hidden message?