The letter from Mirabelle to Eirwyn feels familiar and comfortable, as if written by two people very familiar with one another. I liked the informality and the sense of relationship, and it really helped anchor this piece.

As someone who's coming into this worldbuilding fresh, it felt a bit much all at once, and a little overly-expository. In the span of less than 1,000 words, we're introduced to seven named characters (Mirabelle, Eirwyn, Cassius, Belenus, Rowan, Gosse, Clementine), plus an additional five worldbuilding concepts (Enchanters, The Order, Laurentinus, Ariels, The True World) which aren't given a lot of explanation or context. And while those details do add an element of authenticity to the letter itself and it's point of view, it's a bit confusing for an outside reader who has no knowledge of these things. Presumably, they will be explained later in the story proper, but it made the prologue a little less engaging than it otherwise would have, not knowing anything about The Order, or whether the "protection of the Ariels" is significant or not, etc.

Overall, I think you've got a great starting point for a prologue (and a larger story) here. You clearly have an entire narrative backstory and setting laid out, which is really commendable. I would recommend focusing this prologue on a more digestible moment with less exposition for the benefit of the reader, and then letting some of those other elements and characters reveal themselves in time. But I think you're off to a great start!