1000 years is a long time to spend in solitude. One plans, one waits. One hates. And in that hatred one found many things to do in preparation for one's prophetic return to wage war with one's sister. And that was to touch the minds of the children of the night, those that could hear their princess' desires for revenge. While it is not documented how closely those thestrals listened, Slicer Jen heard. She heard from the time she was picked on by the other thestrals for not having wings, to the time her cutie marks came earlier than everypony else's. She heard until the moment Nightmare Moon was defeated, and heard no more. Or so she thought.
This is a spinoff story detailing the history of Slicer Jen as it pertains to her introduction in chapter 40 of bobbybrony's Dying Embers.