Puzzle - Keyed Cypher

On 5/9/18, The @immortal4tress Twitter account posted the following:"The Sage of Shadowdale hath sent another clue! 'A MONSTER FIERCELY UNDUPLICATED:' And more to ponder on the backside of said scroll: '17, -2, -5, 7, 6, -4, 14, -8, 19, 13, 0, -10, 21, 8, -10, -8, -2, -3, 9, -14, -11, 13, 13, 8, 15, -5, 2, 6, 0, -15, 13, 17, 14, 24, -2' - @immortal4tress"

Solution
In order to solve this, two things had to be done first:
 * 1) The Dead Drops all had to be retrieved from the ten gaming stores. Unfortunately, only nine of the ten have been retrieved at the time of writing, but we have an educated guess as to what the tenth one will be. We'll get to that later.
 * 2) The other Yelp Reviews had to be deciphered to come up with the clue: PLACE SECOND CREATURE AS CODE KEY THEN YOU START YOUR ABC

Step 1: The Cypher Key
Based on the above, we determined that the second creature was from the second gaming store reviewed (Game Kastle), so that keyword is "GARGOYLE".

Based on the clue provided in the Yelp reviews, it was determined that it was to use a Keyed Caeser Cypher, and "GARGOYLE" was the base key. Removing the duplicate key, that made our full key:

GAROYLEBCDFHIJKMNPQSTUVWXZ

Step 2: The encoded text
The encoded text is actually based on the first part of the clue provided, the letters "AMONSTERFIERCELYUNDUPLICATED". By taking each of the letters and shifting them by the numbers provided, you get a new encrypted string:

RKJUYPSJYVEHXMBQSKMGEYVKPOGJ

It is worth noting that there are 35 numbers but the string is only 28 characters... We are missing 7 characters, which is presumably the "monster fiercely unduplicated".

Step 3: Deciphering the String
Putting the above key and encoded text in the Keyed Caeser descrambler, we get the string:

CONVERTNEWGLYPHSTOPAGEWORDAN

...which is INCOMPLETE because it's missing seven characters. At the time this puzzle was posted, the 10th monster - OWLBEAR - had not been retrieved from the gaming store in Winnipeg (see Dead Drops).

This is where it got complicated... We had to reverse engineer the result and determine what the missing letters could be. Assuming that the next letter was "D" (to complete the word "AND"), it would reverse-decode to an "O". So we were looking for a seven-letter monster which started with an "O", and we guessed OWLBEAR.

That being said, if we use the source string as "AMONSTERFIERCELYUNDUPLICATEDOWLBEAR" the full encoded text becomes:

RKJUYPSJYVEHXMBQSKMGEYVKPOGJOHYSSYP

...which decodes to...

CONVERTNEWGLYPHSTOPAGEWORDANDLETTER

...or, in English terms...

CONVERT NEW GLYPHS TO PAGE WORD AND LETTER

At the time of this writing, the purpose of this has not been determined. It is believed to be the key to decoding the crest glyphs being released on social media.