This story is a sequel to Mistress of the Dark
This is also a sequel to That one strange story about a donkey., but the description below is all you need if you haven't read either prequel.
Moondancer and Moonlight Raven have become unlikely friends, despite one being as preoccupied with science as the other is with mysticism. But when an artifact that blurs the line between magic and technology unleashes one of the more unique threats Canterlot has ever faced, the two must set their differences aside (and seek the help of a certain Doctor) to save the day… provided they don’t annoy each other to death first.
This was my entry for Ocalhoun's Big 250k contest, and obviously riffs on That one strange story about a donkey. I feel obliged to note that certain details of Eugene's character are not intended to insult or trivialize others; they're just part of the hand his character was dealt. Cover art assembled from here and here.