This one is a bit different because I'm doing the Let's Meet before really having the character 100% fleshed out. This means I could use your help in maybe brainstorming some of the missing details.

Razor is an Elite in the zebra military. This is the highest ranking field officer. All Elites are extremely skilled combatants with a wide array of tools at their disposal. They have a mastery over their bodies and know how to exploit the weaknesses in yours. I would say that Xenith from the Fo:E story was well on her way to having this level of fighting prowess. I haven't revealed it in the story yet, but the zebra leader Nyx fought in the bayou was an Elite. If the balefire egg hadn't gone off when it did, he could have actually beaten her.

Elites have two main subordinates. Commanders and Loyalists. The Commanders are field officers who do not usually work directly with the Elite, but have their own squads or platoons. The Elite decides what the Commander's mission will be and they go off and do it. Loyalists are individual soldiers who have a life oath in allegiance to the Elite. They often act as the Elite's agents or extensions of their sphere of influence since the Elite cannot be in many places at once. Also, if an Elite is killed, the Loyalists will exact vengeance upon whoever killed them (and their family). Elites often work alone, but they have the freedom and option to command any group of soldiers they please. They are typically well experienced, having completed multiple active duty tours in other roles.

In the case of Razor, he had a rival Elite who he wanted out of the way (the zebra government does not get in the way of these disputes). He arranged for his rival to be in command of the infiltration mission into the Equestrian bayous, as a convenient way to get rid of him via the plague. Razor has a greater motive concerning the plague, though. He wants vengeance against Nyx. It is just a coincidence that Nyx herself got involved in the early spread of the plague, as this was not part of his plan.

The reason Razor wants vengeance on Nyx is because of what she did three years ago at the Royal Guard ceremony. The zebras had planned a major assassination, since the ceremony was being held in a public park with low security and most of the heads of state and many high-ranking military officers would be present. The assassination does not go as planned because Nyx strongly reacts to the first two shots and explosively annihilates 23 of the 24 zebras with some powerful, unknown spell. This puts Nyx under heavy observation as potentially dangerous, but what matters to Razor is the fact that his son was one of the assassins. And since he was killed in a fashion similar to a grenade in a watermelon, there was no body to bury. Razor is pissed, and he wants Nyx to suffer for what she has done. Just killing her isn't enough. So he's contemplating this, and meanwhile, Nyx is still a somewhat high profile pony who is not the easiest target.

Razor has studied up on Nyx and her history. He knows that her origins are as Nightmare Moon, but that she is not really Nightmare Moon as in trying to take over Equestria and bring eternal night, and all that. Razor knows she is Nightmare Moon in body only. But this means she is also strong, if the legends are to believed. Razor is an Elite, so he figures he is still able to win in a fight, especially since Nyx doesn't seem to have much if any combat experience. Razor waits patiently, looking for the right opportunity to ensure that she suffers adequately.

Not long after Razor moves to Equestria in hopes of finding a vehicle for his vendetta, he meets a pony in Zebratown asking around to find who has the strongest feelings against Nightmare Moon. This gets Razor's attention, and the two form a partnership. Razor agrees to provide the tools for the pony's plan in exchange for being able to witness the end of Nyx. Razor has access to the forbidden archives back in the zebra capital and arranges for a bioweapon to be seeded in a zebra military unit infiltrating Equestria. The pony wants this for some reason, in order to put Nyx through the suffering which Razor will find satisfactory.

Razor is suspicious of pony ally, naturally. He doesn't know why this pony would betray what could one day be one of his nation's strongest military assets. And the pony does not tell him about what he plans to do after Nyx is dead. He just reveals the specifics of what she will endure, and this satisfies Razor.

Razor believes the pony will torture Nyx to death or to the point where she will wish for death. This satisfies Razor's vendetta, but he still is wary of Storm Shadow. He arranges the plague to be seeded, conveniently disposing of his rival at the same time. He meets with Storm Shadow to give him the Black Book (which he requested). At this point, the Black Book goes where it pleases. It is very much like the Ring of Power from Lord of the Rings. The consciousness within it has its own agenda apart from the zebras who are merely tools for it. Razor provides the book to Storm Shadow in chapter seven of my story. This might also be a good time to give him the canister of Pink Cloud which he will need for the trap.

After their meeting, Razor departs to observe the spread of the plague. He travels to the bayou and watches from the shadows. He is there to make sure that the plague takes root. Which it does, in Withersberg. Chances are good also, that Razor came across the balefire clearing, after the bomb goes off. He does not run into Nyx or her platoon however, as they are in Gatorton at that time.

Later, when the town is completely overrun with the infected, Razor is quite surprised to find Nyx of all people show up with a sizeable military force and take on Withersberg directly. Razor watches the battle unfold, and notices the infected pegasi flee to spread the plague elsewhere. As fate would have it, two infected ponies run into the woods toward the end of the battle. Their nest has been destroyed, so they are fleeing in hopes of spreading elsewhere. Razor notices them coming toward him in the forest, and Nyx is in pursuit. Razor beheads the infected ponies and hides in the shadows. Nyx catches up only to discover that the two infected which she was chasing have been killed by someone else. Then, everything goes black.

Razor has knocked Nyx unconscious and tied her to a tree with chains. She wakes up to discover that she has been captured. She attempts to free herself, but the chains are strong. Razor reveals himself. Nyx is still wearing the horn ring which Storm Shadow gave her to boost her efficiency. It hides her true feelings from her, so she is more aggressive than usual. She places Razor in a shield, so they are both bound now by each other. Razor taunts Nyx and tells her that he intends for her to suffer for what she has done. There's a moment where they can actually have a dialogue while both are trying to escape. Razor does something odd to his body and he begins to shimmer and slowly push through Nyx's shield. Meanwhile, Nyx is cutting into the chains that hold her. She's pissed, and she begins to focus on Razor's body that is still partially inside her shield. Razor feels himself heat up, and quickly counteracts the spell with his own dark magic. Nyx is nearly free when Razor escapes his own bonds. Nyx tells him she will kill him. Her eyes flash, and her horn flares. Razor realizes he must better prepare for this fight if he is going to win. He flees into the woods just as Nyx breaks her bonds. She gives chase, but he is gone.

The details of this fight are not set. Everything might change about how they fight, but the intention is that Razor is introduced to Nyx and he tells her why he has a vendetta against her. They do not fight long, and in the end, Razor retreats while Nyx goes back to the others.

Eventually, Razor learns that Storm Shadow has hidden his true agenda from him. While he does intend to make Nyx suffer and die, it will be to produce a weapon of incredible power which he could defeat the zebras with. This will not do, so Razor decides to deny Storm Shadow his prize by killing Nyx himself in advance. He tracks her down to a point when she is traveling to the Everfree Forest. Before she gets there, he engages Nyx and her friends in combat. He's ready this time, and the fight is intense. During the fight, Daisy is injured and will retire from the military as a result. Ultimately, Nyx will kill Razor in this battle, which ends his arc.

Ah, been looking forward to this one, love discussing the villains

A possible way for them to meet is that Nightwatch was a POW at the start of the war and was inducted as part of a program to recruit sleeper agents. Nightwatch could have exhibited strong anti-monarchist feelings so was deemed suitable for indoctrination.
Razor could have been a part of the program and so during interrogation got to know Nightwatch on a personal level. Razor himself could be a bit of rebel in his own circles and privately resentful of the Caesar and his staff, so the pair have something in common.
Nightwatch is playing the Zebras all this time and sees Razor as a possible stepping stone to achieving his goals.
After a while the Zebras agree to exchange prisoners and Nightwatch is returned home. Nightwatch has agreed to aid Razor in his efforts, leading to him helping Razors assassin unit infiltrate Canterlot. When that goes south Razor comes to kill Nightwatch blaming him in part for his sons death. Nightwatch however sees this as an opportunity and redirects Razors anger onto Nyx and sets his own plans in motion.

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I really like the POW angle. If he was captured for a while, it would totally justify his wartime stress disorder and motivation for making the memory orbs.

I can just about imagine it, Nightwatch being imprisoned in a zebra cell, scratching drawings of alicorns on the wall along with some obscure ramblings.

In particular, if Razor was somehow in charge of interrogating him, it would lead to that information exchange which is needed for this plot thread to move forward. I imagine one of Nightwatch's first statements would be: "I know how to kill an alicorn."

If Razor learned that Nightwatch had plans against Nyx already, then he would be intrigued and ask for further details. He would likely then arrange for Nightwatch to be released with instructions to go do his worst. Nightwatch would then tell Razor what he needs from the zebras and everything would move forward from there.

Not bad...

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I'm interested to hear what ideas you had on Razors personality.

He strikes me as a ruthless individual but he's also intelligent and not confined by the irrational superstitions of his peers; he knows Nyx was borne of Nightmare Moon but sees her only as a powerful foe to destroy, not a goddess or daemon. I guess he is a product of Zebra martial thinking, taking a route of cold, practical logic when it comes to fighting a battle, shaminsm and magic are simply tools to be used. Yet we have his grief clouding his judgement, he's got an anger boiling beneath the surface that interferes with this mindset, leading to mistakes and errors of judgement.
I'm wondering how he views those under his command, those that have sworn loyalty to him alone, does he view them as expendable for his goals? Or are they as committed to his vendetta as he is?

This is why I love villains, they can be perhaps more complex than main characters driven by skewed morality, simple raw emotion or pain of past experiences.

Yeah, Razor is a pragmatic soldier. I've read the first third of Immortal Game, and I see General Esteem as being a fair bit like Razor, if that helps. As a zebra, he's going to have different values of course. His culture is pretty alien to what we're used to seeing with the ponies. But for the most part, yeah, pragmatic soldier, willing to go to extreme lengths to get the job done. The vendetta against Nyx is a cultural thing as well, being that I've depicted zebras as having a high degree of superstition, tradition, and likely vengeance for wrongs suffered. Razor stands out as less superstitious than the rest of his race, as you've noted. I'm sure he's viewed as very odd as a result. As a rational person myself, I find that irrational people are confused by me every bit as much as I'm confused by them.

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That rational mind would probably help him gel with Nightwatch a lot easier when they first meet. Both are highly driven individuals so when given a common purpose they'd make an excellent team, if not a highly distrustful one