Alternate source: http://dream-phoenix.tumblr.com/post/37483238837/a-special-request-thin-shroud-of-dusk-has

A Christmas present for MarbleYarns and her glorious adventure of Captain Luna.

Accompanying story from the tumblr source:

A thin shroud of dusk covered the city. A strange time it was – too early for street lights to be lit, but dark enough for ponies to hurry to their houses. A strange time indeed.

A pale silver circle in the sky – Lunas moon, was blocked by jet black clouds. The pegasi weather team had issued a torrent for the night. But I hadn’t even planned anything tonight.

All the days chores were finished before the sunset. I was relaxing near the fireplace with a plateful of dandelion sandwiches and a hot cup of my favorite herbal tea. There was nothing in Equestria that was going to drag me away from my sofa!

But just a moment before I was going to nibble my evening snack, I heard a trail of hooves tapping somewhere outside, followed by a gentle knock on the door.

“Commin!”

Who might that be at this hour? And minutes before the rain!

I opened the door and peeked outside.

She was tall with a slender build. Her entire figure was covered by a dark cloak and a hood but I had no doubts that underneath it there would be a regal posture and a stern look on the face.

“Miss Marble?”, exclaimed the unknown night guest with an unusually soft and smooth voice, and continued before I could even nod, “I have rather a delicate request for you.”

Her long (longer than any unicorn would have) horn glowed with majestic blue light and a folder flowed from her cloak. It was grey with some mark – unknown to me. But what would surprised me most was a stamp – a dangerously red TOP SECRET stamp on it.

“I think you are just the right pony for this issue”, she said to me as I stood mesmerized.

The mark on the book’s cover: