Dusk and Dawn

Dusk And Dawn is a My Little Pony: Friendship Is Magic fanfic by 2Percent. It is based on an alternate ending to the second episode - Nightmare Moon is not defeated by the Elements of Harmony, and instead takes over Equestria with a brainwashed Twilight Sparkle, now known as Eclipse, as her general.

Tropes used in Dusk and Dawn include:

A God Am I: The position Eclipse would occupy if she overthrew Nightmare Moon and took control of both the sun and moon.

"Twilight, the barrier between day and night, and the ruler of both. Poetic, wouldn't you agree?"

Berserk Button: Eclipse hates Celestia even more than Nightmare Moon. The mere mention of her name was enough to make her kill somepony.
Bittersweet Ending: The story starts and ends with Nightmare Moon still in charge, and now she knows the Elements of Harmony are still around and will likely take action. However, the Dawn Militia is now aware that Eclipse is arguably a Well-Intentioned Extremist who shares their goal and has offered them an alliance.
Cold-Blooded Torture: Nightmare Moon is a fan of this, usually with Eclipse on the receiving end. She also gets off on watching it, evidenced when she orders Eclipse to torture Spike.
Deadpan Snarker: Eclipse. Enemies, allies, Nightmare Moon, it doesn't matter. She really pushes her luck with the latter, too, since she knows she's irreplaceable.
Downer Beginning: The story begins with the Dawn Militia's hideout in Manehatten being raided thanks to information leaked by a spy, and the city being placed under martial law.
- Also of note, Nightmare Moon won seven years ago. This is mostly just mentioned however, so it doesn't count for the trope.
Immortal Life Is Cheap: Eclipse can regenerate any damage done to her body in seconds. This allows Applejack and Trixie to wail on her with as many weapons and spells as they can and Nightmare Moon to inflict all the Cold-Blooded Torture she wants.
- The best example being when Nightmare Moon essentially rearranges Eclipse's insides. This would normally be fatal, even for somepony with a healing power if they have to manually activate it. Good thing Eclipse's appears to be automatic.
Kick the Dog: When a young unicorn servant implied to be new to her job freaks out after Nightmare Moon loses her temper, she orders the unicorn thrown in the dungeon.
- Nightmare Moon ordering Eclipse to torture Spike.
Major Injury Underreaction: After suffering a brutal lashing at the whim of Nightmare Moon, Eclipse heals herself and calmly asks the massage attendants to get back to work. They're more freaked out than she is.
My God, What Have I Done?: Sort of. Eclipse claims that her crimes are a necessary sacrifice to save what's left of the world from Nightmare Moon's tyranny. She doesn't appear to feel remorse until the end, where she claims that she could've never brought herself to do them back before NMM's rule, and that she hates what she's become (along with what she once was, both queens... and pretty much everything else). For more on that, see Tragic Villain below.
Precision F-Strike: The story mostly uses the same euphemisms as the show (hay for heck/hell and such), but Eclipse gives us this gem when she freaks out at the end:

"Why does everything have to be so fucked up?"

The Night That Never Ends: Since Nightmare Moon got what she wanted, here we are.
Replacement Scrappy: Trixie in-universe, sort of. As Eclipse points out, Trixie is only the bearer of the Element of Magic only because she can't fulfill that role herself anymore, and furthermore, Trixie is unable to use the Element to its full capacity. However, Applejack vehemently denies the label "replacement."
The Stoic: Eclipse is described as being cold and condescending, and almost always in control of her emotions. She doesn't falter when snarking to anypony, up to and including Nightmare Moon herself, and can calmly think or magic her way out of any dilemma. However...
- Not So Stoic: Mention Celestia in her presence and she'll probably kill you. And if she doesn't, she'll just go on a rant about the former queen's shortcomings.
  - She also drops the facade and shows real pain when Nightmare Moon tortures her again at the end of the story. And when Spike gets brought into it, she cracks completely.
Shoot Your Mate: Nightmare Moon grows frustrated when Eclipse proves resistant to her torture, so she delegates it to Spike instead... and orders Eclipse to carry it out. In this case it's not a test of loyalty, but instead simple punishment for her insolence.
Shut UP, Hannibal: Trixie and Applejack each make one of these at Eclipse's Hannibal Lecture. Only Applejack's works to any extent.
That Mare Is Dead: Eclipse does not take kindly to being called by her previous name - Twilight Sparkle.
"The Reason You Suck" Speech: Eclipse makes a lengthy monologe mocking Trixie for her past actions, mistakes, and weaknesses. Trixie retorts by casting an elaborate binding spell and telling Eclipse that she already knows this, and that she had been training for years to fix it. Then it gets Subverted when Eclipse shrugs the spell off, points out that Trixie's violent reaction only supports her speech, and proceeds to verbally beat her into a Heroic BSOD that requires Applejack's intervention to break.
The Starscream: Eclipse wishes to overthrow Nightmare Moon herself.
Tragic Villain: Eclipse undergoes a sort of Heel Realization at the end of the story, and despises what she's become by following Nightmare Moon's orders. Whether this shows hope for a brighter future or how far she's truly fallen, and that she's incapable of redemption is up to the reader.
- According to Word of God, it's the former.
Villain Protagonist: Eclipse is arguably the main character. What's also arguable is whether she's actually a villain or just a really dark Anti-Hero.
Well-Intentioned Extremist: Eclipse.

Eclipse: "It will be worth it in the end. You can't honestly believe the lives of a thousand ponies outweigh the sun and the well-being of hundreds of millions?"
Applejack: "Don't you use your fancy mathematics to muddy the issue! You're killin' good ponies, and that's that!"

Woobie, Destroyer of Worlds: Eclipse, in keeping with her Villain Protagonist status. Her final lines are what sells it.

Eclipse: "I never wanted this," she murmured. "Why does everything have to be so fucked up? Damn Nightmare Moon. Damn Celestia."

You Leave Him Alone: Eclipse delivers a less heroic variant of this ("He's mine!") when Nightmare Moon goes to torture Spike, marking the first time she's truly stood up to her queen beyond sarcasm and snark. It doesn't work out exactly as planned.