With focus, comes strength
 
*Writer’s note:  
-I assume each season of the show represents roughly one year’s time of passage. In the present, Siren is in her early 20’s, putting her in this flashback at around 15-16. Yes she is a blank flank at that time.
 
-My headcanon for Luna’s personal guard was that while she was imprisoned on the moon, her Guard went into a state of hiatus.  
I also differentiate between a Royal Guard who works at night (a Night Guard), and the more specific Lunar Guard, who are Luna’s personal body guards. After Luna’s banishment, the Lunar Guard didn’t have anyone to protect and their unit shrank in numbers. Most of these individuals went on to pursue other life purposes. However, a select few of loyal family lines within this guard volunteered to pass down their legacy to each generation, in perperation for the return of Princess Luna, and revive this honorable duty. And these Lunar Guardsponies were known for their strict warrior culture, trained from an early age.
 
-This Lunar Guard never witnessed the transformation of Luna into Nightmare. In the show we saw during the flashback that Celestia banished Nightmare Moon with no witnesses to observe her transformation or banishment. Through the centuries, the story of what happened to Luna evolved from historic fact, to word of mouth myth (Kind of like how no one’s ever heard of it in the pilot episode). Eventually, these loyal Guardsponies reached a social status that we (on Earth) would associate to something akin to warrior monks. A duty greater than their own lives.
 
Siren was born into the generation which would welcome back a Princess who was lost to time. No pressure, right?
 
But wait, Danger, then why is Siren attacking Luna now if she was trying to be one of her guards only 5 years ago!?
 
That is exactly the question I want you to ask ;)