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This cipher has me vexed. FOOTOOLTLTLBDCLOLTLTJBFTOCFTOOOCDC. There appear to be too many Os(and no other vowels) to be a transposition cipher. The coindidence index is 14.75, which is quite high, but with the repeating LTLT and the triple O no substitutions have made sense.

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    $\begingroup$ Welcome to Puzzling SE. Is this cipher from an outside source? If so make sure you have permission to post it on other locations and include the source. $\endgroup$ – gabbo1092 Nov 19 '18 at 15:41
  • $\begingroup$ A friend created it and gave to us to solve, so we should be ok. $\endgroup$ – user3233665 Nov 19 '18 at 17:03
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    $\begingroup$ To add a touch more context, this message was provided to our DnD group by the DM at the bottom of a summons for help. All of us have been stumped. $\endgroup$ – user3233665 Nov 19 '18 at 18:00
  • $\begingroup$ Only 8 different letters are used. Is this a cipher in octal? $\endgroup$ – Weather Vane Nov 19 '18 at 23:01
  • $\begingroup$ Could it be related to the rulebook for the version you play? Like a shorthand for a pre-existing set of words like chapter titles, character names, a known phrase...? $\endgroup$ – feelinferrety Nov 19 '18 at 23:06
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Okay I think the message is

Don't trust the Kenku.

Reasoning

As you pointed out, it looks like the letters are actually broken into pairs rather than encrypted singly (see the frequency of the pair LT) so let's break the ciphered text up into pairs of letters

FO OT OO LT LT LB DC LO LT LT JB FT OC FT OO OC DC

This looks rather likes a Polybius square cipher where we are using letters instead of numbers. I played around a bit to try to determine the order of the letters on both axes and determined that a very nice thing happens if we use the order CABOT on the horizontal and FJOLD on the vertical axis (also Cabot Fjold sounds like it could be a place in DnD).

In particular, the Polybius square looks as follows

    C A B O T
F  A B C D E
J  F G H  I  J
O K L M N O
L P Q R S  T
D U V W X Y

To decipher, for any individual pair of letters, you take the first letter on the vertical bold axis and the second letter on the horizontal bold axis and find the letter in the grid with the corresponding coordinates, so for example LT becomes T.

Translating, our ciphered text becomes

DONTTRUSTTHEKENKU = DON'T TRUST THE KENKU

This is backed up by the fact that a Kenku is a character from Dungeons & Dragons.

Edit:

I've just realised "Cabot" and "Fjold" are mentioned in the comments so this must be the answer.

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