Hello there. I remember reading this story a while back. I don’t remember much other than that twilight was a Valkyrie, or Guard or something badass for Luna I think. I also remember she was the best and most fearsome out of all the other members of her unit or something like that. And there was a huge battle. I know this isn’t a lot to go on and any help would be appreciated. And it’s not Crimson Night

I think I know the one your talking about, but for the life of me I can't remember, I'll look for it and get back to you.
Edit: I found the one I was thinking about twilight is an orphan who becomes Lunas protege, but it seems pretty dead, it litteraly is called "valkyrie".
Ps hope this was helpful.

7091870
This makes me think of The Right That Returns, though that was humanized versions of ponies...

7091873
Nope that’s not it. I remember one scene when there was a pony in the throne room with bad intentions and Twilight swooped down from the ceiling and killed him

7091881
Ok sorry that I couldn't be of much help, but have a nice day (Or night depending on timezones) and enjoy your Christmas.
Ps can you tell me the story if it is still alive if you find it, I might read it.

Are you sure its not The Right That Returns? I remember something like that happening in there.
Although you may be referring to this story. https://www.fimfiction.net/story/104188/valkyrie

It is the first one that shows up when you search “Valkyrie”. There are a few that show up.

Trying to think of the stories I know offhand that have Twilight working for Luna or something like that. I feel like I know the story you mean, but can't quite pin it down. Something that I think was similar was Twilight Sparkle: Night Shift or maybe My Little Dynamite or perhaps The Moon's Apprentice.
I feel like I know the exact story you mean, but goddamn I just cannot think of which one. I'll keep digging.