Oh so he is an adeptus custodes nice

7127968 I'm curious...Is he one of those naked guys in the ITEHTTSD?
I don't know much about WH and most of my knowledge is from that and online vs debates.

And since this is rather early, they aren't exactly xeno=dead? Though, one could argue that these ponies have Terra origin.

7127968 Yes, from before the Emperor was placed on the golden throne. Before you ask why I know that, It's because his cloak is red not black.

I haven't read anything in the HH era books about the custodes, but I do know a few things about them. They have incredibly long names, with a new name added to their own after every campaign/heroic action they complete. As a result they only use one or two names at a given time that they are known for. Also, I don't think the custodes take vows of silence, they just don't bother talking to anyone who they view as unimportant to their duties i.e. The Emperor. I think they also engrave their full names on their armor as a record of their deeds, though that just might be something Gothik made up for his fanfic Renegades. That said I could totally see a custodes in this situation adopting the name Sun Guard and adding it to his own.

I can't help but feel this is the Custodes that they retconed Ollanius Pius with.

7343489 He took the vow by himself it wasn't a Custodes thing.

FOR THE NEW LUNAR REPUBLIC

9061116
For Equestria's true queen

"Princess he just murdered two of your guards!"

"Dont worry about it, hes fine, in fact, I'm going to offer him a job without any prior knowledge of him or investigating into the pony that literally just murdered two of my guards!"

What

I am am a little disappointed that are custodes is not that of the lovable kitten but it is still a good read none the less so far

10186891
Or is he? *evil snicker*

After reading this first chapter, and some of the comments, I will agree that it's odd that Celestia just flat out accepted him despite having killed two of her own guards.
Or at the very least, the way her response came out and her behavior so far. The action of her accepting Sun Guard despite so readily killing her own isn't far-fetched but I feel like the way it was written wasn't executed quite right.

If he only injured/knocked out the guard, then her current response might be fine.
Since he killed them though, taking a life is a permament thing. Now, if this was the sort of Celestia who, in some stories I've seen written that went in-depth about her past, was more rational and less attached to her ponies in her past during her sister's banishment, then it would be reasonable for her to take on Sun Guard as a personal guard to make up for what he has done (killing two of her guards, seen as a resource at this point in time). Her response though should be more distant, and not of someone that is accepting a friend with open arms. Furthermore, her behavior before meeting Sun Guard should likely be one more akin to sorrow and self-despairing/denial perhaps?
I feel like this then could help lead to a much better scenario, and even have Sun Guard's words be the reason for Celestia being who she is 1,000 years later...
Not wanting to be drowned in sarrow in her Sister's banishment, she starts focusing herself more to the betterment of her ponies and, eventually, herself after a time. This would follow closely to the words Sun Guard said that his Emporer mentioned; "Remember those that you rule over, and your responsibility to them. The right thing isn't always the easiest thing." Though she would likely focus more on the first part, which in her own interpretation lead her to become more caring and compassionate of her ponies in the future.
To help emphasize to her the importance of Sun Guard's previous charge though it would likely be a good idea to have her sense some sort of mystical or high power that is drifting off of Sun Guard, the remnants of his Emporer's power that still cling to him due to his time under his service. This way it at least gives his words some merit and weight, and doesn't seem like Celestia is just trusting his words out of nothing.

This is, at least for myself personally, how I think this scene could have gone better. Still though, the whole concept you got going here in general is looking pretty damn cool. Hope you decide to continue writing this again in the future.