I haven’t thought out all of the lore of this in great detail, Luna isn’t killing Celestia-gone-bad, she’s just defeating the Daybreaker form and freeing her sister from Daybreaker’s influence. I don’t THINK this needs a semi-grimdark tag, for Daybreaker basically going “Ack, you got me!” but I will happily change it if told to, or someone else can.
 
I was inspired by this famous engraving of the duel between Alexander Hamilton and Aaron Burr.  
https://www.flickr.com/photos/internetarchivebookimages/14598442187/
 
Rendered and composited in Blender cycles with experimental 2.79 denoising, post-procedded in Photoshop CS6  
Daybreaker and her armour are by me, as is Luna. Original pony heads by Juicedane. Luna’s jacket is derived from a Daz asset  
Used assets:  
CC-0 Longsword by heraSK https://www.blendswap.com/blends/view/73583  
CC-BY Flintlock by Felis http://www.blendswap.com/blends/view/70830  
CC-0 Meadow Scene byRedFrost https://www.blendswap.com/blends/view/78528