Here be spoilers!

This cipher is theoretically solvable without any knowledge of the Empire setting, it’s just a bit fiddlier.

Each number is a letter, and each slash is either a space or a punctuation mark.

Bryn’s introduction gives you a few words which are likely to appear somewhere in the letter.

You could try looking for: Cynestan, Bite & Anvil

What’s the highest number used, and what’s the significance of that number?

The number 383 occurs several times, and is the highest number used.

383 also happens to be the current year.

What’s something with a clear connection to the years since the foundation of the Empire, which there are 26 (ish) of?

The numbers 381, 382 & 383 all occur at least once, but there’s no sign of 377, 378 & 379.

FULL SOLUTION:

Connect each number to the reign of one of the Imperial Thrones, starting with the First Empress, all the way to Empress Lisabetta, along with the First & Second Interregnum.

Match each reign/interregnum in chronological order with the letters of the alphabet (minus J, Q, X & Z) to get the following cypher:

• A: 1 - 21

• B: 22 - 34

• C: 36 - 89

• D: 71 - 91

• E: 92 - 102

• F: 103 - 120

• G: 125 - 167

• H: 168 - 179

• I: 180 - 199

• K: 200 - 209

• L: 212 - 234

• M: 239 - 248

• N: 249 - 257

• O: 257 - 281

• P: 281 - 300

• R: 300 - 318

• S: 324 - 329

• T: 331 - 346

• U: 348 - 368

• V: 371 - 373

• W: 374 - 376

• Y: 380 - 383

Which gives the following text:

Cyenstan,

I agree to your offer.

Ten doses of bite.

Ten rings each.

Wintermark is too far to travel.

Meet me in Anvil.

Saturday night, nine, Winter Solstice.

I suggest Navarr camp away from prying eyes.

Boar’s Head.

I am a cambion and will wear red.

Come alone. Be vigilant.

Tammen von Holberg

This cipher is, to the best of my knowledge, utterly unsolvable without access to particular bits of information on the Empire Wiki.

Once you’ve figured out the method, it takes a bit of legwork to get to the full decoded solution.

Flavia’s introduction gives you a few words which are likely to appear somewhere in the text.

The solution will instruct you to “send a Winged Messenger,” try looking for that phrase.

On first glance you might assume that each block of letters (eg. “RTTWC”) can be decoded into a word, but this isn’t the case. They’re actually individual letters.

Each block of letters needs to be translated into a number, which can in turn be translated into a letter.

The key here has something to do with magic.

The following all relate to the same letter: CB, DM, OP, NP, RL & VOTD

The following are all “stand-alone,” they are each the only block of letters that relates to a particular number/letter: VOY, IMS, SOTBC & TM.

Each block of letters is the initials of a ritual in Imperial Lore, or a spell:

• HOTM > Hand of the Maker

• VOTD > Voice of the Dead

• LP > Leonine Pavilion

These are grouped together by their magnitude, with spells counting as 1.

• HOTM > Hand of the Maker > 8

• VOTD > Voice of the Dead > 1

• LP > Leonine Pavilion > 20

Then each number relates to the letter with that position in the alphabet. A=1, B=2, C=3… (There are no magnitude 25 rituals, so 26=Y&Z.)

• SOY > Skein of Years > 8 > H

• VOTD > Voice of the Dead > 1 > A

• LP > Leonine Pavilion > 20 > T

And with a bit of legwork, you get the following text:

“Well done. Now, send a Winged Messenger to Vincenzo di Sarvos in Tassato Regario, The League, including the word “endless” and your name, and await further instructions.”