So I've got a shiny new Rare Soul, and I haven't been crushed by the reality of getting another 3* Corrupted Owl Cultist just yet.
Is the unit determined when I get the Soul, or when I use the Soul on the Book?
It's an important question. Say I have one of every unit currently available already (or more likely one of each unit I actually want).
There's no reason to use further Souls, so one might think they could stockpile them for future unit releases. And if the unit is determined when it's dropped in the book, that's a possibility.
But if it's determined when I get the Soul, then there's no reason to stockpile because all of those Souls are already determined to not contain the (hypothetical) new units.
This is really key when it comes to the Trooper purchased souls. You can only buy one a week, and stockpiling points doesn't really gain you much once you have enough to buy everything as soon as it locks.