In light of the Paris Fan Festival version of the trailer, with Y'shtola being addressed as "Matoya", it opens up a new and interesting reading of the prophecy - particularly the third line.
In its various translations:This seemed to be just a metaphor, but now I'm thinking it might refer to a literal event: Matoya's soul somehow being bound to Y'shtola.(E) "As scion joins to stock of withered bark"
(J) "The foliage of a dying tree grafted to the tree that replaces it."
(J) "From a rotten tree a branch is taken, to be given to it's successor." (post #13)
(G) "But the tree of history shall bloom"(can "Baum der Geschichte" be translated a different way? "Ancient tree"?)
(F) "Like green growth sprouting from a doomed trunk"
Putting all the translations side by side, they seem inconsistent about what's actually happening though.
The Japanese version seems to make the most sense. English seems back-to-front (why graft a new cutting to a dying tree?) but perhaps got swayed by the tempting wordplay of "scion" - does that carry over in translation, or is it only the English name for the organisation that carries that double-meaning?English: the scion (grafted branch or shoot) is attached to "withered stock" (the roots).
Japanese: the scion is a cutting from a dying tree, grafted to new stock so some part of the old tree lives on.
German: doesn't seem to match the others
French: a "doomed tree" putting forth new growth; no mention of joining things together. I suppose it could follow that the new growth is taken and grafted (as in the Japanese) but it's not said outright.
So anyway. Assuming Japanese is correct: part of an old tree grafted to the new one that replaces it, so part of it lives on.
It certainly seems very appropriate for the currently-theorised situation that Matoya may have transferred her soul into Y'shtola's body.
In which case....
You've used the English version of the translation so even if it's just a metaphor, it might need to be the other way around. "Like a cutting from an ancient tree, we will graft a piece of the Darkness onto ourself."
If the third line is not a metaphor, it becomes clearer again. "As what happened when Matoya placed herself within Y'shtola, so shall we take Darkness within ourself."
I also have to wonder about the prophecy itself: the "anachronistic journal". (How does the Japanese version attribute this? Or doesn't it say anything? It hasn't been included in the translations.)
Urianger seems to have shifted from studying prophecies to actively making them as an astrologian.
And what might get a found journal labeled "anachronistic"? The writer's vocabulary seeming a few centuries out of line with the apparent age of the book...?