MLP/Undertale crossover.
Not a PWNY-verse story.

Long ago, the Monsters and Humans fought a great war. The Monsters lost, and were sealed in the Underground. It would take the power of seven human souls to break the barrier that kept them imprisoned. They only had six. And so the Monsters waited and hoped, praying that another human would find their way to the Underground. Some thought the seventh soul would give their King the strength to break the barrier. Some dreamed of a human who would be their friend, and save them all...or had nightmares of one who would bring back the war.

...but nobody came.

...but somepony did.

Eons after the war, with no way to tell how much time had passed above, a tiny blue filly with wings and horn fell into the underground, calling for her Mama. None knew the poor creature's nature or origins...but her soul was powerful with magic, even more so than a human soul.

None can know what will come...of this little 'Woona'.

Cover art by Sanyo21

Nightmare Moon returns and is hit once again with the Elements of Harmony rendering her a mischievous little filly.
Much to Nightmare Moon’s annoyance, Anon thinks she's cute as all hell and is the only one willing to put up with her antics and evil, so Celestia tasks him with reforming the bundle of misery and mischief.

Cover Image is by EvetsSteve, may he rest in peace.
Chapters 1-21 were converted into prose from my greentext stories with the help of Nav/WhatMustIDo

One cold morning, a small grey filly crested the low hill at the edge of her family's farm, sneering down at the smoky swamp below as she sucked furiously on a squeaky yellow pacifier.

She was just old enough to understand that the arrival of her little sister Maud made her the "big girl" now... and she didn't like that one bit. Even worse, she'd heard mommy and daddy whisper that another was on the way.

But down there, in the swamp, was the means to end this madness. All she needed was to grit her teeth, toddle forward, and take it.

A cute and silly drabble inspired by the cover art, and proofread by hawthornbunny!
Featured on FimFiction 8Dec.2016-9Dec.2016
Also featured by Seattle’s Angels

After the rousing success of her first party, bringing joy to her parents and sisters, Pinkie Pie left her home to teach that joy to others. On her first day in Ponyville she stumbles across a struggling Sugarcube Corner, and resolves to help out the business.