What hides between Shadow and Light?

[Bloody_Eugene]

So, my suggestion is to try 13 or (especially) 9 as a seed in the decrypting. I don't really understand how it works so I might not be big help with it. It's 3am here so I'm going to bed.

Thank you for your analysis!!! New things and ideas are welcome!
You know what? There are 117 letters in the text and 117= 13 x 9 !
I'm pretty focused on the number 13 also because the lines of numbers are of 13 numbers each.

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BTW,

* I think I found something *

Do you remeber:

I'm still trying to track down the second line and i've found this:
8...6...9...6...11...8...13...10...15...8...11...4...9
..-2..+3...-3...+5...-3...+5...-3....+5...-7...+3...-7...+5

I'll try to write it in another way:

8 (-2=) 6 (+3=) 9 (-3=) 6 (+5=) 11 (-3=) 8 (+5=) 13 (-3=) 10 (+5=) 15 (-7=) 8 (+3=) 11 (-7=) 4 (+5=) 9

Please note that the signs are -,+,-,+,-,+,-,+,-,+,-,......
And that the numbers to add/subtract are all prime numbers.

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and

This morning I've found another thing: in the first row of scrambled letters there are 81 letters, and in the second row there are 36 letters.
Now 81=9x9 and 36=6x6.
Isn't it strange? We can build two square of letters (I built them horizontal, but maybe it can also be vertical) like these:

YNGPPUZFV
KJZIDSTCW
IZVYIDFYV
QOPSBAVJU
IUNJVPLIF
RMHMZQDFA
BMNWMMLRY
SWAQAZRSX
QEDMJWHSG

and

VJBSFJ
VTCSVV
CPFIMG
MUNSOC
OQGRRJ
UCAVIS

Well, I worked again with this and prime numbers and +,-,+,-,+,-
the series of prime numbers is:
1, 2, 3, 5, 7, 11, 13, ....
if we put the signs on the series we will have:
+1, -2, +3, -5, +7, -11.

Now I tried to apply the 1st line on the second 6x6 square of letters and what I found?
V , J , B , S , F , J
+1, -2, +3, -5, +7, -11
------------------------
W , H , E , N , M , Y


It really seems that "When my" is the beginning of a question that may lead to the answer we have to submit!
But I was unable to go further. Nothing seems to work on the second line "VTCSVV". Nothing. Damn. This puzzle is driving me insane.

EDIT: Sorry for my ignorance, just checked "prime numbers" on Wikipedia and "1" is not prime. So my theory doesn't stand up, and now i think I've not found anything. "When my" could be a huge coincidence of letters, nothing more.