So here is an idea I came up with that has the first chapters mostly fleshed out. If you don't know what Youjo Senki is then here is then watch the video above for a quick summary. I will be combining the Novel, Manga, and Anime but on things that contradict or overlap I will defer to what is closer to the source material.

Being X complains how even after being in another world for years Tanya has not shown an ounce of faith and plans to make things harder for her. Another god points out that even though strife can lead to faith their usually is a reason for it, such as to protect their family and loved ones. Tanya has no connections to others in her second life so faith cannot grow with such a poor foundation. The god then suggests another world, more peaceful but still dangerous where Tanya can learn how to care for others and from there, learn faith. Being X agrees.

(Go to 32:00 for relevant scene.)

Tanya is in the final testing of the Elinium Type 95 Operation Orb when it beings to have a magic meltdown. Being X tells Tanya that he is disappointed at her lack of faith but that he changed his mind and will give her one final chance. Being X does not intervene and Tanya is killed in the resulting explosion and crash, something that is bound to have interesting butterflies in the world. (I would like for Tanya to have more of her combat experience from fighting in the Rhine. So maybe I will move the timeline up to when Tanya fights Sue and have Being X help him defeat Tanya.)

Tanya wakes up as a unicorn filly in Equestria. (Some possible names so far are Pixie Dust, a reference to her call-sign, or White Silver, Tanya's second Mage name.) Luckily ponies grow up faster then humans so Tanya quickly begins to study her new surroundings. One of the many changes she would find is her magic. Her horn acts much like an operation orb would, allowing her to cast her old spells. However everything has to be cast manually without help from a computer (Tanya will not be capable of the feats she does later on that require Being X's blessing or having a multi-core orb.) After a few years pass Tanya finds herself enrolled in school and is around the same age as the CMC. Her life is monotonous and boring compared to the military training and combat in her second life. Tanya trains and studies in her free time but making sure not to stunt her growth like last time. (Tanya won't be able to use magic to dope herself due to not knowing what effects it can have on her as it can be potentially lethal to her new form. In will become available in the future with more study.)

The time for the festival arrives (Episode 1) and Nightmare Moon appears. Tanya goes to prepare, covering herself in soot to disguise herself and grabs a spear (Either from one of the unconsious guards, the guardhouse, or a weapon she owned for practice) before flying after NMM

After NMM "destroys" the Elements Tanya arrives and challenges her, inadvertently preventing Twilight from realizing the spark to restore the Elements. NMM mocks Tanya for being a child with Tanya responding by calling her an old hag. Furious NMM accepts the challenge. Tanya states the challenge to be Trial by Combat just as a magic bolt pierces through a window and hits NMM in the eye. Tanya urges Twilight to run before fading away, revealing it to be an illusion. The real Tanya is flying in the sky a distance away. It was a perfect shot, unfortunately Tanya is still just a child and so her magic isn't strong enough to be lethal especially to an Alicorn. NMM bursts out the window and charges at Tanya at top speed, half blind. Tanya soars higher into the clouds and the dance begins.

While NMM has powerful magic Tanya still was once an aerial mage, she dodges NMM attacks with skill and decoys before luring NMM into an ambush in the clouds. When NMM chases after her Tanya drops down and pierces NMM wing followed by a kick to launch her to the ground. Before Tanya can celebrate her success she is pulled down by NMM magic and sent crashing to the Earth. Tanya tries everything to slow down before powering up her shield and bracing for impact. Tanya breaks through the trees, her shield protecting her from suffering scratches. Tanya hits the ground and rolls before coming to a stop. Tanya survives but ends up with a broken leg (possibly some broken ribs/internal bleeding.) Stunned by pain Tanya can only watch as NMM walks up to her, both wings now broken. (Tanya has no pain killing magic so has to push pass it the normal way.) Tanya watches fearlessly as NMM prepares to finish her off but at the final moment a magic shield cast by Twilight stops the attack. NMM turns to unleash her fury on Twilight only for Rainbow Dash to buck her in the face, interrupting the spell. Applejack and Rarity then arrive, working together to hogtie NMM. Fluttershy lands next to Tanya and gives her some emergency medical care. Pinkie Pie launches a barrage of Cupcakes against NMM and buries her in them. While the others celebrate defeating NMM Tanya and Twilight remain focused.

NMM bursts out of her bindings with a magic blast shocking the party. Twilight prepares to cast another shield but before NMM can cast her magic a spear is launched into her chest. NMM bursts into shadows and fades away. The sun slowly returns and the party slowly begin to make their way home. Celestia arrives and praises them much like in canon. Twilight sadly tells her of the "destruction" of the Elements. Confused, Celestia asks what happened to NMM. Tanya spits out blood and says they did it the old fashioned way but that she managed to escape. (Celestia is relieved that Luna isn't dead but her plans are in ruins so I'm not sure what she would do next.) Twilight returns to Canterlot to prepare for NMM eventual return and find another way to defeat her permanently. Tanya now has to explain how she got her injuries to her parents.

I missed a lot of details but this is the basic outline.

• So a big change from canon is that the Equestria will have to fight the first seasons villains without the Deus Ex Machina of the Elements. Luckily Discord will still be imprisoned so Tirek won't get any help from him.
• I want Tanya to eventually play some political games with the nobles but I also want her to interact with events in Ponyville. Suggestions would be helpful. One idea I have is her being a cousin to Blueblood with her parents hoping to keep her out of the game and have a normal childhood.
• I will be taking canon events but putting new spins on them both major and minor. For example the ticketmaster episode can be Twilight returning to invite everyone but then having to search and find Tanya who was disguised.
• I had a lot of problems on what to do with NMM. The first version had her completely dead but that would cause problems with Celestia. Tanya would definitely have gone for the kill and had no real nonlethal options against an Alicorn. This version solves that and makes NMM a threat in the future.
• In this fic I'm making it so Alicorns and ponies in general are resistant to most magic and blunt impacts. However Tempest Shadow lost her horn from an attack from a Minor Ursa, Slash and Piercing damage. An Anvil has dropped on Twilight and her horn didn't break so it must take a blade with power to do so.

with all certainty Pixie Dust!
It makes some sense but I think she would seek maximum training efficiency anyway and she would ignore the feelings much more, in fact she would want to grow in life as a sunset style maga.
She already has some experience she can start to create a spell to do the calculations that were made by a computer.
I believe she must use an extremely well-planned and powerful style to defeat NMM in submission, put terror into her and then accidentally purify her on the basis of using pure logic to destroy her beliefs, but if that does not work she can give time to Twilight take your spark and steal the moment.

The rest of the fic can alternate between Pixie being forced to live an ordinary life and spoiling this (besides being very angry) and moments of action provoked by her style and the conflicts of the own series, besides of moments of intervention of entity X .

I'm crazy for more of this anime so I'm waiting for your fic.

6463401
Rushing ahead man. I haven't finished yet. So thats one vote for Pixie Dust.

NMM will ask Tanya for her name. As she is disguised would she say her name is Tanya or White Silver?

Tanya

your overall idea is very good and I would like our German loli giving moral lessons on our incident ponies. It would be nice if in the end the six manes were removed for political games as well, would make sense, not to mention that Celestia would keep an eye on Tanya after all that. But I reinforce q our protagonist has q: 1-always walk armed 2- keep up logic and despise the feelings while keeping the demon q we love. 3- Give fear to the ponies! I would love it if she scared the nightmare off of Luna.