I KNEW IT! I KNEW IT! hahaha......nicely written
Your story is very mysterious - naturally. It's interesting, though, that you don't even name your main character. It's fascinating.
I would recommend removing the author's note at the top. It may be useful, but it may discourage readers. You could contact your reviewers individually to ask them questions, if you wish.
The figure of a tall woman wearing flowing robes crossed across the darkened hall of the fourth floor. It's difficult to wear flowing robes. They flow independently from the wearer, if that makes sense. Remove 'wearing' and replace it with 'in'. Also, you don't need to say 'crossed across'. 'Crossed' is enough by itself.
The conversation that follows is interesting. It's relatively clear who the speakers are, but I almost picture them as being in the dark, consulting in secret.
You shouldn't capitalise the first letter of the speaker after a quote. Here's an example:
“You have a good explanation for your tardiness, I take it.” The voice continued.
"You have a good explanation for your tardiness, I tale it," the voice continued.
It's a little error, but do be careful of that.
The woman though f a moment trying to remember. You just need to make that 'for' and add a comma after 'moment'.
It continues to be intriguing that you never name anyone until the end. Even when your main character talks about the new recruits (her spy status is curious), you don't use their full names.
The twist at the end is surprising - I suppose Minerva was only a spy in this era? There has been mention in canon of her relationship with Tom Riddle, so the connection you've made is a valid one.
Minerva... a spy... now, that is a twist! I look forward to reading more!