Beyond the Narrows

"Cossacks! Oh God, Cossacks!"
Corporal Allison Church looked at the bloodied Marine approaching the perimeter of her squad's camp, and she froze. She had faced down Covenant, fanatical Insurrectionist militias, and their equally as fanatical but even more skilled regular counterparts, but nothing had prepared her for this. His helmet was gone, his face looking like that of a horror movie victim, with blood gushing from a gash that ran down the side of his head, where his ear once was. His uniform was equally gruesome, no longer the gray color of standard UNSCMC fatigues, but instead crimson from several more gashes around his neck and chest. 
But it was not the gore that disturbed her. It was the fear. There was a look of fear in his eyes, not a fear of death by plasma fire or gunshot, but a deep-seated fear that if he didn't stop running, he was going to suffer a fate far worse than death. She didn't even recall his name, but what she did recall he was from Glabetov, a planet that had Cossacks running amok back when it was still under Innie control. While rebel propaganda spoke and depicted them as noble horsemen running down the animalistic Oonskies, pro-UNSC sympathizers like this private told a different story. A story of medieval barbarity. Of burned settlements, beheadings, and orgies of violence and pillage that humans had supposedly left behind a millennium ago. But the Cossacks, no, these Innies playing dress-up as a long-dead tribe, had decided the way to progress towards their freedom was to regress in their humanity.
And now they were here and had just cut down the patrol of Marines she had just ordered out to maintain a perimeter. 
The young Marine collapsed at her feet and grabbed onto her leg, whether out of exhaustion or out of abject fear, she could not say. And he started pleading with her.
"Corporal, if we don't get out of here now, they're going to kill us all! They're going to do unspeakable things to the people they capture, I've seen it with my own eyes, they're savages, please, we have to leave now! I don't want to die at their hands!"
She looked down at the Marine, his eyes welling up with tears as he continued to babble about the terrible fates that awaited if they tried to stand and fight. She had seen nothing like it before, not when fighting fellow humans anyway. This was the behavior of someone who had just seen a Brute rip an ODST in half and was now turning towards him, not someone who had been ambushed by a bunch of bloodthirsty in rebels in funny clothes. But she couldn't shake the disturbance of his fear. But she had to shove it back down. Now was not the time to run scared. She looked down at him.
"Private, we're not going anywhere. Now you can lay down and let the rebels in silly clothes tear your throat out, or you can stand and fight and die like a Marine."
She shook him off his leg and turned around to the temporary tents that held her squad.
"Everyone, get up now! We're under attack!"
Turning back around, she now saw the Marine, struggling to his feet, begin shuffling towards the line of tents. But as he passed out of her field of vision, she began to hear his footsteps recede, and get quicker. Snapping her head round, she saw him running like a man possessed away from danger. She involuntary grunted as she withdrew her pistol and fired a round into the air.
"Private, stop now! I said stop damnit!"
But nothing seemed able to stop him. Her Marines looked at her, unsure if they should chase after him or get ready to defend themselves.
"The MPs will take care of him, get ready, we've get an assault coming on!"
As she began to step to help one of her gun crews get an M247 set up, she heard the whinnies of horses and a loud "Hurrah!" from the nearby treeline. Looking back, she saw them. These apparent demons in the shape of men. Of the fourteen men, about eight wore the uniform of what she had found to be frontier troops, green-crowned caps with a khaki pullover and trousers with high jackboots. The other six cut a striking image in comparison. On their heads sat cylindrical black fur caps with a red top. Their jackets were not pullovers like their frontier companions, but modified civilian blazers, with the red and black embroidery of a folk shirt peeking out of the collar and a military belt with cross-straps holding their ammo pouches and swords, with a pair of khaki baggy breeches completing the ensemble. In all their hands shone the blades of bare metal sabers that glimmered in the morning sun as they raised them and galloped forward.
Time seemed to slow for Church as the horsemen approached, yelling like furies. A crack of a bullet passing by her head pulled her out of her trance, and she suddenly realized they were on her and her squad. The thwacks of sabers cutting into men, fire of pistols from the horsemen, wild fire of MA5s from her own squad, whooping of the rebels, and panicked cries of their horses filled the air. One Marine attempted to grasp one of the Cossacks by his coat and pull him from his horse, and was rewarded for his effort with another riding by behind him and slashing him across the nape of his neck.
Soon, the remainder of her Marines too began to run for their lives. She shouted in desperation "Stop, damnit, stop!" as she fired her pistol wildly towards the swirling throng of horsemen. As they ran past her, she stood firm. Today would not be the day she ran from a bunch of raggedly dressed rebels. The cohort turned towards her, and towards her fleeing squad, and began their charge anew.
Click. Click.
She quickly dropped her clip, and reached for another. But as she brought it up to load into her M6, she felt a strike on the back of her neck, and fell to the ground from the force. Then the feeling of hooves trampling her as the bones in her torso fractured from the impact.
Laying on the ground, Church found she could barely muster the energy to raise her head to look towards her squad. Maybe her stand had allowed them to escape into the treeline where their speed could not be utilized. But as she looked, her heart sank
As the horsemen fell upon her squad, yelling what she could make out as "Slava Kozaki!", their sabers flashed once more, slashing the backs of those Marines who attempted to flee. Those who attempted to fight were run through at swordpoint or gunned down by pistol-wielding rebels. A few Marines, numbering three or four, threw up their hands, dropping their weapons to the ground. As the noise of battle died away, the only noises that remained were the tramp of hooves on the soft dirt, the heavy breathing of the horses, and the pleadings of the Marines.
"Please don't kill us! We only wanted to fight the aliens, not you!" the highest ranking of them, a Lance Corporal, Church's second, pleaded.
One of the horsemen, wearing the khaki uniform of the border troops but the black fur cap of the Cossack horsemen, rode up to them, pistol in hand, as the remainder of the horsemen encircled the three Marines, obscuring them from view.
"Alright, we won't kill you. But if you resist, I'll have you end up in the same place as the rest." he said, motioning towards the dying Marines lying in the sod, groaning from their gunshot wounds and saber cuts.
"Fine, anything, just please don't kill us."
"I'm a man of my word Oonskie, I said I wouldn't kill you, I won't. Bohdan, Roman, Bohun, tie 'em up and blindfold them."
"You got it sir."
Three of the horsemen dismounted, one fetching a coil of rope from his saddle, and began binding the three Marines.
"Alright, now let's get out of here." said the one in charge
"What about the rest sir?"
"Doesn't matter. They won't last long enough to tell a tale. And if they can, they can tell the story of the Cossack victory. Slava Kozaki!"
"Heroyam slava!" responded the cohort.
"Now let's get out of here."
Church wasn't about to let these rebels get the last laugh. Grasping the dirt in her gloves, she groaned as she struggled to her feet.
"I'll tell them of a band of silly-dressed rebels who look like they're out of the last century!" she sputtered out.
The horsemen looked over, a bemused look on some of their faces. 
Their commander spoke up "Don't pay her no mind. We don't have much time. Hyah!"
He turned his horse around and they rode off, the sound of their hooves slowly fading.
Church felt her legs turn to gelatin, and as stars filled her vision, she collapsed facefirst into the dirt.
****
"Any survivors besides the ones we found?"
"Nothing." responded the regimental coroner
"Think their babblings are any worth? I mean, horsemen with sabers? That's something out of a fever dream."
"I doubt it. Come here." the coroner motioned towards the body of one Corporal Church if the ID tags were anything to go by.
"She's got deep lacerations, as do the rest. And it's not from torture, it's as if someone either dragged a very sharp blade at high speed down their body or hit them very hard with the cutting end of a blade of some sizeable length."
"Hmph. You really think these horsemen exist?"
"Think Captain? I don't think, I know. Nothing but sabers at high speed could have done something like this. And each of those Marines tell the same story. As do each of these." he said, motioning with the latter sentence towards the neatly laid rows of bodybags.
He turned back to his datapad, and marked Corporal Church as KIA, confirming the electronic notification to next of kin, one Leonard Church. 
"Next!" he said, motioning for his assistants to remove her body and bring the next onto the table for identification.