Luna rushed towards her sisters private quarters. It was a very rare occasion that Celestia sent a messenger who used phrases like 'severe danger' or 'fate of Equestria at stake'.
Perhaps it was just a prank of her sister, and nothing noteworthy had happened at all, but she couldn't take the risk.

She soon entered Celestia's rooms and found her sister near her desk with a letter in her magical grasp.

"What is it, sister? Has Equestria run out of cake?" Luna teased.

"Read this," Celestia replied and hovered the letter towards Luna.

"What, no response to my cake teasing?"

"Just read the letter."

Luna took the letter with her magic and started reading.

"Oh, a letter from Twilight. It is always nice to read something from... WHAT? AN ALIEN?"

Luna stared at her sister, who just said

"Read on, it gets better - or worse."

So Luna continued reading:

"So... alien... warrior... a human from an 'Imperium of Mankind'... Wait, human? Like those creatures of ancient tales? Impossible...
He doesn't like magic? Well, then he will really enjoy his stay here...
What else? Most ponies are afraid of him. Hmm.
Modified super-warrior?
Fighting daemons of Chaos?"

After she had finished reading, she added:

"A real Twilight-report. Very detailed. And most interesting. Perhaps I should pay this Antonius Varus a visit..."

"No, Luna, not on your own. We both will meet him tomorrow afternoon, I already told Twilight that we will come to her library.
In the meantime, promise me that you won't set a hoof into the library until tomorrow."

"I think you are exaggerating, sister. According to Twilight's letter, he seems not to be hostile."

"Perhaps not, but I won't take any risks. And it is not just him I'm worried about. We wouldn't stand a chance against a pan-galactic empire.
We must find out if it really was an accident that brought him to Equestria or if there is a plan behind all this.
Now, Luna, your promise?"

Luna sighed.

"Okay, sister, I promise I won't set a hoof into Twilight's library until tomorrow afternoon."

"Thank you, Luna."

"No problem, sister. Now, if you will excuse me, even with such a creature in Ponyville, I still have my duties."

Luna left her sister's quarters and headed to the barracks of her Night Guard. Of course, she would keep her promise and not enter the library, but taking a look around couldn't harm anypony.

The night was still young when Antonius Varus had almost finished the book he had requested from Twilight.
As he had expected, the ponies did not only use the language of the Imperium, but also the letters. Some looked a bit mangled, but he could still read them.

Although the book answered some of his questions about ponykind and even went beyond pure anatomy, it also raised new ones.

The ponies were, all told, standard horses, if you ignored the horns and wings at least; all inner organs were where he had expected them to be.
That meant, if he had to fight them, he should concentrate on the head, heart or the upper spine for maximum efficiency.

Strange was the fact that their more 'private parts' were completely hidden inside a sealable skin fold, which was a hint that the species originated in much colder regions. The rest of their bodies hinted at a moderate climate. It didn't fit.
Perhaps, the history of Equestria he would get to know tomorrow would give him information about the origin of the ponies.

Antonius had a closer look at the different pony kinds:

According to this book, Earth ponies were especially versed in growing plants.
The reason for this ability was - magic. So, not only the unicorns used magic, but the Earth ponies, too, although in a more subconscious way.

The pegasi were flying impossibilities: Weight, weight distribution, wingspan, wing surface... they really should not be able to fly.
But they flew anyway, because of - Antonius had guessed it - magic, again subconsciously.

The unicorns were a special case, as they used their magic deliberately with the help of their horn. It was some kind of focal point for magic.
An interesting fact was that the horn was not completely integrated in the skullcap, it sat on its top instead. So, if a unicorn lost its horn due to an accident - or an attack - it would still survive, but lose its magical abilities.

All told, every pony on this planet seemed to use magic in some way, it was an inherent part of their species, so Antonius was surrounded by psykers of varying skill levels.
Not really a pleasant thought.

He closed the book and looked over the maps. Luckily, this planet had a stable magnetic field and the ponies even used the same cardinal points as mankind, as Twilight had told him.
One of the maps covered the area around Ponyville and included the farm he had seen today, 'Sweet Apple Acres'. The map was not very detailed, but it was enough to give him an overview.
The other maps showed more of his surroundings, such as the capitol 'Canterlot' and a town called 'Cloudsdale', but the details here were even fewer.

Just a moment... 'Canterlot'? Wasn't there a place with a similar name in an ancient terran legend?
Yes, there definitely was. So, not just legendary creatures, but also legendary places.

The last map Twilight had given him was some kind of world map.

After studying the maps and committing them to his memory, the Space Marine started to think about his situation and the status of his mission to reunite with the Ultramarines.

He had opened up a new source of information in order to find a way back to his chapter. If this source would turn out to be valuable, time would tell, but even if it really did, wasn't the way he socialized with the ponies treason? They were xenos, after all. But they also were his best chance to complete his mission.

Did the end justify the means? And were the means he used really treason and heresy?
Was he doing the right thing? Antonius was not sure about this.

The Codex Astartes allowed joining forces with aliens to fight a greater, common threat, and the ponies were supposedly enemies of Chaos. There was no active threat at the moment, though, the ponies were just his best source of information.

Cast onto a planet full of apparently friendly xenos who looked like creatures from ancient terran legends.
This particular situation was not covered by the Codex.
Antonius had to improvise.

He needed time to think. He needed to pray to the Emperor for guidance, but not from here, not from this alien town. The hilly area where he had made first contact was more fitting for this. Perhaps, he could even spot a known constellation in the night sky and determine the location of this planet, although the probability for this was miniscule.

Antonius put his helmet on again, sneaked to the library's front door, opened it and closed it again quietly after he had left the building.

It was a clear night outside. The town was lit by a full moon and lots of stars were in the sky.
Antonius took a deep breath and started walking through the town in the direction of the farm.

He had just passed the first buildings as he got the feeling that something was watching him. His right hand went to the handle of his bolter as he turned around and looked at a shadow cast by one of the houses of Ponyville.
There was something in the shadow... like a shadow within a shadow. He knew there was something, but even the auto-senses of his helmet, scanning the electromagnetic wave spectrum from infrared to ultraviolet, had problems making it visible. His targeting system could not outline it, it could just place a maker over it to visualize its presence.

Was that some kind of daemon?
Unlikely. A daemon wouldn't just stand there and watch him, it would attack.

Well, Antonius would find out what it was.

"Step out of the shadows and show yourself!" he said, ready to use his bolter.

The creature that stepped out into the moonlight and became visible was indeed not a daemon, it was a pony, but it was a pony unlike any he had seen before:
A dark blue mare with a horn and wings - an alicorn, the first one he saw. Her color was a nice variety compared to the pastel-colors he had seen until now.
She was bigger than the other ponies - about half as tall as the Space Marine - and slimmer, reminding him of the sign above the entrance to the 'spa'.
She wore a black necklace with a white crescent moon insignia, which was also her cutie-mark. On her head was a black tiara.
True eye catchers were her mane and tail - they were not just hair, they looked like patches of the night sky somehow attached to her body and flowed in an non-existent wind.
From the descriptions he got from Twilight, it had to be Princess Luna.
She greeted him with a warm smile:

"Greetings, Antonius Varus from the Imperium of Mankind. I am Luna, Princess of the Night, Guardian of Dreams. I hope you are enjoying my night."

Her night? Yeah, sure.
Should he tell her that he knew how solar systems actually worked?
Better not. Messing up his first encounter with one of this land's leaders would be unfavorable.

He further wondered why she knew his name. Telepathy? That was an unsettling thought.

"Greetings, Princess Luna. I am indeed enjoying this night. I am looking for constellations that could help me determine where this planet is located. But how do you know my name?"

"Oh, Twilight Sparkle sent a letter to my sister and me, telling us that you are in Ponyville.
I can assure you, she only had the best intentions."

"I don't mind that. It was a sensible procedure."

"Good," said Luna, then her smile faded a bit as she continued:
"About the constellations, Mr. Varus, I must tell you that you won't have any luck. You see, this is not the actual night sky. The actual night sky is veiled and I put up these stars instead."

She did WHAT?
Antonius really had to ask her some questions about that:

"Veiled? Why is it veiled? And how?"

"As for the 'how': it's magic, but not my magic.
I honestly don't know why the veil was put up in the first place. It is like that as long as I can remember.
I can influence the veil, but I was told never to unveil the actual night sky."

Before Antonius could say anything, Luna continued:

"There is one star you can still see through the veil, though.
Would you follow me to the outskirts of Ponyville? You have a better view of the star from there."

The Astartes was still processing this information.
A veiled night sky? Just one real star visible? The rest of the stars put up by Luna? Then it really was kind of 'her night'!
He just said

"Lead the way," and followed her.

They made their way to the southern border of Ponyville, where Luna pointed at a star which's light was different from that of the other stars.

"You see this star, Mr. Varus?
It keeps shining through the veil and never changes its location. We call it the 'Southern Star' and use it for navigation. It is... special.
Have you ever heard of such a star?"

Antonius had a feeling that this star was important, but he did not exactly know why.

"I have never heard of such a veil in the first place.
As for the star, without knowing how this magic you mentioned works, I could only come up with wild assumptions.
I need more information...
Princess Luna, what are the risks of unveiling the actual night sky? At least the part of it where you can see this star?
I think that would help a lot."

She looked down, deep in thoughts.

"I'm not sure of the risks... perhaps a small patch... for a short time... that should keep the risks at minimum."

She looked up again.

"I will discuss this with my sister. This decision is too important to be rushed.
If we agree on breaking the veil, I will inform our subjects to avoid confusion, then we will have a look at the real universe.
I can't promise anything now, though."

"Sounds reasonable. "

There was a short silence after this conversation, broken by the Space Marine:

"Did you come here to have a look at me and show me this star?"

"No. I mean, not only. One of my duties is to patrol the Everfree Forest for dangerous creatures and keep them away from my subjects.
Do you want to join me? I could tell you about those creatures when we encounter them."

That actually was a good idea. Antonius would be able to determine the thread-levels of the local wildlife that way.
Furthermore, he would get the opportunity to balance all of today's friendliness with a fight, so he said:

"Yes, thank you for your invitation. "

They started walking towards the Everfree.

That did definitely not go as Luna had planned it.
Instead of catching a glimpse of the newcomer, she was walking beside him now, on their way to her guards who were waiting at the edge of the forest.

Sure, she did not enter the library, just as she had promised, but in spite of that, her sister would not be happy.
No. Not at all.

Celestia had always been rather cautious when dealing with new situations.
In this case, she was probably overcautious. Antonius Varus made a quite sensible impression on her. If the rest of his 'Imperium' was half as sensible, there would be no problems.

Inviting him to her nightly patrol was bold, she had to admit, but she was curious about his fighting style and combat prowess.
Judged by his equipment, he would probably do quite well, even though she didn't know what his weapons were capable of.

However, there was another reason, too:
He was surrounded by ponies, yet, he was alone, the only one of his kin. Most ponies were afraid of him.
Luna knew this feeling all too well.

She had to help.
Of course, Twilight and her friends would try to gain his friendship, but Luna was not sure if those nice ponies would be able to handle a real warrior, if they would be able to stand the violence he represented.
She was sure she could handle this, she also had fought terrible wars, after all. If anypony was suitable to become his friend, despite his martial traits, she was the one.

They closed in on the edge of the forest and Luna's Night Guards came into view.
She had selected her best two soldiers for this night's patrol, just in case, Moonlight and Blackwing, a bat-pony couple who complemented each other perfectly. She was a happy-go-luck melee expert, equipped with retractable hoof-blades, he was a level-headed, outstanding markspony with a hoof-mounted crossbow.

They bowed before Luna when they saw her and she introduced them:

"Mr. Varus, this is Moonlight and this is Blackwing, two members of my Night Guard.
Ponies, this is Antonius Varus from the Imperium of Mankind. He will join us tonight."

Moonlight smiled at the Space Marine and said:

"Hey, reinforcements! Welcome to our squad!
You don't mind us calling you Antonius, do you? It's a little more personal that way, squad-mate."

Now that Luna thought about it, picking Moonlight might not have been her best idea, but she didn't have a chance to foresee this situation.
Luckily, Antonius didn't sound angry when he answered:

"If it's common among your kin, I will accept it."

Moonlight continued with a bigger smile:

"Tony?"

"Do not get carried away, pony," Antonius said, a latent threat in his voice.

Before she could say anything else, Blackwing put his hoof over her muzzle.

"Hello, Mr. Varus. Glad you are willing to help us. Don't mind her, she's just... like that."

"I'll do my best. Now, please give me a second."

Antonius stared at each of the three ponies for a few seconds, then he said

"Thank you. I'm ready now."

Confused by this strange behavior, Luna asked him:

"Excuse me, Mr. Varus, but what did you just do?"

"I reconfigured my targeting system, so that it recognizes you as allied units."

"Targeting system?"

"This device scans life forms and objects and classifies them as imperial, allied, neutral or hostile."

"Oh, that sounds convenient."

"Hey, wait a second!" Moonlight interposed.
She had obviously been able to free her muzzle again.
"It's nice that you can 'classify' things, but can you also fight? What weapons do you have?"

"I thought my weaponry was pretty obvious."

He was right with that.
Especially that large sword with the serrated edge was an eye catcher.
How could you fight with such a monstrosity? Those teeth had to get stuck everywhere!
Perhaps it was more of a ceremonial weapon?

Moonlight continued:

"Yeah, but as squad-mates, we should know each other's weapons.
Blackwing here has a crossbow.
I have my blades.
Princess Luna has her magic.
What do you have?"

"Bolter, MK Vb 'Godwyn' pattern, equipped with standard bolts.
Chainsword, MK XI 'Hell's Teeth' pattern.
Standard combat knife."

Moonlight looked a bit confused now.

"Err... I heard 'sword'. And 'knife'.
That's good, I guess.
'Bolt' doesn't sound bad, either."

Blackwing face-hooved and Luna decided to end this conversation before Moonlight could cause a diplomatic incident:

"Now, if everypony is ready, we should get going."

Antonius took the black box he had called a 'bolter' in hand, Moonlight and Blackwing nodded.
Then, they started their patrol. Luna took the lead, the Space Marine to her right, and the batponies walked behind them.

Immediately after beginning their patrol, Luna noticed that Antonius was well skilled in that kind of activity.
He showed great attention; there was no noise, no movement in the darkness that he missed.
Intriguing was the fact that, despite being such a giant, he moved with barely a noise.

Until now, they had not encountered any hostile creatures, the forest was peaceful and quiet.

Suddenly, Luna felt the beginning of a nightmare in Ponyville.

"Stop, everypony."

Everyone stopped, and Luna turned to Antonius:

"I sense a nightmare in Ponyville. I must carry out my duty as Guardian of Dreams now.
Please wait a few minutes until I'm done."

"No problem, Princess, there is something I could do in the meantime.
Excuse me now - and don't follow me."

After he had said that, he vanished into the forest.
That was a bit strange, but Luna had other things to worry about. Rarity had a nightmare and needed assistance.

It turned out that Rarity was not the only pony to have nightmares that night. Those nightmares were clearly influenced by the story Antonius had told them, as they contained daemonic creatures and a lot of violence.
Most of Twilight's friends needed Luna's help, except for Rainbow Dash.
Despite not sensing a nightmare in the blue pegasus' mind, Luna took a look:
She was happily kicking... creatures, wearing some strange armor that had great similarity to the armor Antonius wore.

Now that she thought about Antonius: Where was he and what was he doing?

"Maybe I should look for Mr. Varus," Luna said once she had finished her dream-duty.

"That might not be the best idea, Princess," opined Blackwing.
"He sounded serious when he told us to leave him alone."

"You two wait here, I will have a look where he went."

So Luna sneaked into the direction the Space Marine had gone, becoming one with the shadows.

Antonius Varus had found a small clearing, still within earshot, but far enough away from the ponies to be out of sight.
He knelt in its middle, holding his sword upside down in front of him, so that its tip touched the ground.

He was still praying to the Emperor for guidance when he again got the feeling of being watched and detected silent hoofsteps to his left with his Lyman's Ear.
Luna. The fact that she had followed him was annoying, but not serious enough to stop praying.
If it was another creature, he still had the chainsword in his hand.

After finishing his prayer, he stood up and looked in the direction of the hoofsteps he had heard.
He could see the same shadow-within-a-shadow as before, put his sword back to his belt and said calmly:

"You followed me."

Luna appeared like the first time they had met.

"Sorry, Mr. Varus, but the forest can be dangerous.
I just wanted to see if you are all right, but now that I have seen this, I want to ask:
What were you doing?"

"I prayed to my Emperor for guidance."

"To return home?"

"Among other things."

Antonius looked into Luna's eyes and saw... compassion?

Before he had time to wonder why she empathized with him, she asked:

"You are feeling lonely, don't you?"

"I beg your pardon?"

"You are separated from your people, on a planet strange to you.
I would be a miracle if you didn't feel lonely."

He had no idea what he should answer.
He would not say that he felt 'lonely', but he was certainly dissatisfied with his current inability to carry out his duty as Guardian of Mankind.

Luna continued:

"I know this feeling. A friend can help you to overcome it."

Oh, for the Emperor's sake! Another 'friendship'-fanatic!

"As I told the pink pony, Ms. Pie, before, I prefer to get to know a person or pony before I start a friendship," Antonius told her.

So, that should stop her. His assumption was proven to be wrong when she said:

"Well, you may not want to become my friend now, but I am definitely your friend.
Do you at least accept my friendship?"

That could simply not be true.
Had she really offered him a sort of one-sided friendship? Because she thought it would make him feel better and less 'lonely'?
It had to be a trick. An inferior motive behind all this.

"What benefit do you expect by offering me this one-sided friendship?" he asked bluntly.

"Perhaps it will become mutual sometime.
Even if it doesn't, it will still be a step towards a better understanding of each other.
Furthermore, I think it's just the right thing to do."

Just the right thing to do? Was this her honest opinion, or a trick to gain his trust?

Antonius looked at Luna.
Damned, those ponies really had big, expressive eyes!
Now that he looked into the blue eyes of Luna, realization struck him:
It was not a trick.
It was... kindness.
A kind xeno!

That could not be, it was against every experience mankind had ever made! Xenos were either malevolent or rather neutral at best, not kind and friendly.
Unless... this whole planet was a new experience to him, so, could this just be another new experience? The first genuinely friendly species mankind had discovered?
He somehow knew there was a decent possibility that this was true, as it fit the behavior of the other ponies he had met, but he was still wary.

He could deny her request, of course, but that seemed... wrong. He would give Luna the benefit of the doubt.
He took a few steps towards her, crouched to be more at eye-level with her and finally said:

"Accepted. I will give you a chance."

Antonius could hardly believe that he really had said this and accepted the friendship of a creature every imperial citizen would call a xeno witch, but he knew it had been... just the right thing to do.

On the one hand, it was a tactical move to gain the xeno's trust, on the other hand, he knew that that was not the whole truth. A part of his mind really had wanted to accept Luna's friendship.

Was that an Astartes' decision?
Antonius was not sure of that, but he was sure of one thing:
It was a human's decision.

Interesting. His usual daily routine had never allowed this side of him to come to light, but despite the transformation, the training, the indoctrination and the conditioning, he was obviously still a bit human at heart.
A somewhat unsettling thought. It could be a flaw, a weakness. He would keep an eye on it.

Luna smiled as she said:

"Thank you. Please, just call me 'Luna' from now on. Ready to continue our patrol?"

"Ready. And Luna... I think it would be fair if you called me 'Antonius'."

They went back to the waiting batponies and together, they resumed their patrol.

Author's Notes:

Introducing: 2 bat-pony OCs, I don't know if their names (or character traits) are already used by someone else. If that is the case, I can assure you that this is purely coincidental.
If you are waiting for a fight, you can look forward to the next chapter!