Hi everyone. I am a new DM taking on an ambitious campaign as my very first, for my friends who are also some first time players. I wanted to make an effort to incorporate their backstories into the world and their adventures. I am using some modules from the radiant citadel and combining some lore i have found on the city of Huzuz in Zarkarah. 

One of my players has a revenge backstory build around some raiders that burned down her village but I want the situation to be more complex. It has been 15 years since the event in their backstory and rather than a random group of raiders, these people have found real power into the political workings of the city and management of the outer territories. Maybe as their own type of thieves guild or a right hand group of mercenaries to to powerful person within the city. I want them to be easy to find but more difficult to find a single person of influence within it and, taking them down should be harder than just overwhelming them with force and killing their leader. Or if they easily kill this powerful person there should be real consequences that surrounds this act that my player characters will have to deal with. 

I am struggling with general writers block and lack of experience to to build something like this out so if anyone has suggestions on where i can find some inspiration or recommendations on what i can do it would really help. 

Mercenaries don't burn down villages for fun. They do it because they're being paid to do it. Which means someone rich and powerful was actually responsible for the attack. So even if that band of mercenaries has acquired the protection of the rich and powerful of a certain area, finding and killing those mercenaries might make the character feel better for a bit, but it won't punish the person who gave the order. And it won't stop that rich & powerful person from recruiting more mercenaries and burning more villages. Finding out who gave the order, getting close to that person, and exacting revenge, will be darn near impossible. That person's wealth and influence in their city means there is an entire political elite and an entire "official" military and local police force at their disposal. The mercenary group is just a side hustle they use when they want to maintain plausible deniability. This would easily be a two-tier story arc at least. Heck, the person who gave the order might even end up being the Big Bad Evil Guy of the entire campaign!

First, you need to know the political and military structure of the city and its surroundings. Making the BBEG the top guy (King, Emperor, etc) would be too obvious. The BBEG rather is a power-behind-the-throne sort of person. After all, it's easier to get away with atrocities when it's not your face that's in the public eye every day. Next, you'll need to decide WHY this person is paying mercenaries to burn villages. Maybe he just wants their land. Maybe those villagers are a certain kind of people that he hates. Whatever the reason, he's not gonna stop until someone stops him. Next, you'll need to map out (like with string on a whiteboard) what the dynamic tensions are between the official military/guards and this mercenary group that has recently been granted a semblance of legitimacy. The official guys are probably not too happy that a gang of criminals is committing horrible crimes and being rewarded for it. This may give the party an opportunity to try to convince the military to stage a coup. 

Again, this is an ambitious and complex undertaking. In Tier 1 (levels 1-4) the party would simply be gathering clues to try to figure out what happened and they will learn that the mercenaries who burned the village are now a semi-legitimate extra-judicial paramilitary group. In Tier 2 (levels 5-10) the party may try to infiltrate that group to gain intel, or they may try picking off members of the group one by one. By the end of Tier 2 they will have learned who actually gave the order. In Tier 3 (levels 11-16) they may gather the remaining villages to stage an uprising, drawing out the mercenaries, and then wiping them out. Or they may infiltrate the official military/guards to stage a coup. Or they may use more direct attacks against the wealthy and influential leaders. But no matter what they do, the BBEG always seems to be one step ahead. After all, you don't attain those levels of power and influence without knowing how to watch your back! 

This story arc would probably end at the end of Tier 3, or maybe at the beginning of Tier 4 (levels 17-20). There should be a fundamental shift in power that exposes those responsible. This needs to be something big! Something that shakes up the entire power structure of the region. Maybe a natural disaster. Maybe the King/Emperor/Whatnot dies and is succeeded by their young idealistic child. Heck, maybe there's an invasion of monsters and the powers-that-be simply barricade the city and refuse to help anyone but themselves. That'll be sure to expose them for what they are. Whichever set up you use, the punchline will be the same - Civil War. The story arc ends with a massive battle in which the forces of good finally face off against the forces of evil. Both sides call in whatever allies they can. Maybe the Bad Guy has made a deal with a Devil for his power, so he's got devils on his side. Maybe the party helped a Clan of dwarves at some point, so they've got dwarven reinforcements. Make it big, make it dramatic, make it cinematic! 

Eventually, the battle will wear away enough of the Bad Guy's defenses that he will be left exposed, with nowhere left to run, and the party will exact whatever Vengeance or Justice they deem worthy. Or maybe they all die in the attempt. Again, this is a big and complex story idea involving a lot of intrigue, skullduggery, diplomacy, and mass combat. It'll require you to do a lot of homework, and prep work, and to maintain clear notes along the way. But if you pull it off, it'll be Epic!