I'm going to expand on this, because I've thought about it a lot! Anastasia isn't numbered, because she never got to ascend, and therefore she could have never been the “Xth Saint to serve the King Undying”. There's been a lot of fandom speculation around when her attempt happened—she worked it in tandem with Cassiopeia, incidentally, which would hint at her developing her technique alongside everyone else and not afterwards. Some suggested she might have tried it first, her failure discouraging others from attempting perfect lyctorhood—but I don't think she was either the first nor the last. It might have been after Augustine and Mercy and G1deon, but before Cassiopeia. We just don't know!
(As an aside, all we know about Cassiopeia and Nigella's relationship is that Cassie was protective of her. They were probably lovers, yeah, but considering God's polycule, the Dulcie/Pal/Cam/Pyrrha situation, and Harrow's entanglement with Ianthe, this is not a series concerned with monogamy. Anastasia hasn't had a single line of dialogue and she's already been shipped with both Alecto and Samael.)
Now, let's turn our attention to the text. The letter could be hypothetically between many people—even from some character we've never even heard from! But let's see what we know from the text itself:
Okay, so it's directed at a woman.
Tomorrow you will become a Lyctor and finally go where I can’t follow.
Okay, directed at someone who presumably became a lyctor. “Finally go where I can't follow” is very rich. “Finally” would imply expectation—it will be the first time they are thus separated, after trying to be each other's companions for long. “Go where I can't follow” could be metaphorical or literal, but considering the lyctors, upon completion, emigrated from the Dominicus system forever and their plan was to go to the Mithraeum very far away (which must have been readied before the ascensions, given there were quarters prepared for everyone including Anastasia, which go unused until Harrow gets there).
I want you to keep this letter when you are far away and think of me and want me and can’t have me, and know that no matter how far you travel,
Okay, this gives us two bits of info: one, they desired each other carnally. Very hot.
Two, the letter is about physical distance. This is not a letter from a cavalier who is going to “join” their necromancer in lyctorhood—a language of separation is never used for lyctorhood, quite the opposite!—and the text tells us in three different phrasings that they're gonna be separated by distance.
You could make an argument that it's all figurative language. I just really don't buy it, because nothing about it works against all the language about union in the series. Cavaliers are never considered to be left behind by their lyctors. And finally,
nor how long the years feel, the one thing that never stays entombed is
So we get. First, anticipation that our lyctoress will find it hard to live for millennia. And then, a reference to what I imagine might have been their passion or love, or just a sex joke (Palamedes manuals it was a love letter)—and it's told through the imagery of what stays entombed.
It's the only bit of the text that makes reference to anything beyond these two people's feelings. One could argue that it's necromantic culture so of course people speak about tombs, but we really hear very very little about tombs in any context other than the Ninth.
It really sounds like it has been recently established—and it's of importance to the writer—that something has been entombed and will remain there forever.
This is all from the text itself. I think Tamsyn was extremely intentional, as she usually is, when she chose what figments to show us from this communication.
Again, for all we know this is a love letter from Teacher to Mercymorn (someone write that fanfic!), but that's not the most likely option. The most likely is that this is about the two people who are most immediately relevant to us and to our main characters (the Sixth, in whose custody the puzzle was, in a series about the Ninth) one of whom we know ascended and therefore got physically separated by distance and eventually time, and the other of whom found that the rest of her life would consist of watching over something meant to remain entombed. Considering Anastasia and Alecto had some sort of pact, and Harrow considers that her role as Reverend Daughter entailed “promise of part in her resurrection”… this could be a letter about their horniness, but it could also be a veiled message about resistance.
#which could mean nothing etc. etc. but my point is that textually there's a lot more evidence for this letter being Cassiestasia than anyone else, certainly more in my view than for it being from a cavalier to their own lyctor!
As an additional note, and purely for the lulz, imagine Anastasia channeling her new House's entire aesthetic to finish that paragraph with my boner.
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