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! 

New GM with new players?

Run one of the starter adventures. Seriously. Lost Mine or Dragon of Ice Spire Peak. Both have built-in tips for GMs. After you've got several sessions under your belt, look at where the party is and whether you can include material from their backstories.

I would say to have a flexible mind where backstories are concerned. Backstory is a story not the game. It is material that can, but not necessarily should be included in your campaign. Most of all, realize that not everybody can come up with a colorful and interesting backstory. It doesn't really matter what happened before the PC decided to become an adventurer. What really matters is how they act with the current situation.

I really like where you are going with this. Going into this part of the campaign the players are already at level 7. The premise for the party to make their way back to this player's homeland is due to the help of a powerful NPC they have gotten mixed up with. The reward they get for doing a favor is information on how to find 1 of the people they know was a part of the raid. This NPC has some nefarious intentions because she is somewhat of a devil and is puppet-ting a situation that will cause chaos to break out in the area (I think end goal she is BBEG).  

Would it be more climactic that if at that point, this person at the top, has already done something to slip up on their power and the payers are coming in at the point where they can both take on some of that tier 2 and 4 level areas. I am hoping that it can be climactic but not take more than 2/3 levels for the completion of the arch. I do want to play around with the consequences of this player character's actions and how their quest to take on this vengeance will cause a chain reaction in one way or another. 

As for set up and organization, what resources or recommendations do you think I should look into for this tiered structure. If i am looking a well established lord or merchant who has power, influence, and the organization to pull this off successfully for years. Is this a structure with them at the top, a shady contact for raiders to do the dirty work, and then someone who spins this discourse into their favor. Then for raiders,  how organized is their own organization structure and how much would they rely on the person at the top? If the motivation is power, would it be plausible that the raider's have their own agenda in this whole thing that they can be persuaded about? How can i reasonably portray the amount of power that they have in this city as they peel aback the layers?

Here's the problem with hiring mercenaries. A Boss sits atop an organized hierarchy. Whether it's a State, or a kingdom, or a corporation, or a guild, it is the structure of the hierarchy through which the Boss imposes their will. Structure and hierarchy are inherently Lawful. If the Boss is using their position and influence to do evil stuff, then it's Lawful Evil. All the myriad people who fill the ranks of that structure are loyal to the Boss because they are loyal to the structure. Structures have their own rules, and traditions, and their own forms of etiquette and expectation. For example, when a new recruit enlists in the US Army, they don't swear an oath to the President, they swear an oath to the Constitution and the UCMJ (military code of law). They swear an oath to the structure. That's what gives organizations their lasting power, the fact that even if the Boss is killed and a new Boss takes over, that new Boss will already have an army of loyal workers to command, because those workers are loyal not just to the Boss but to the structure that the Boss commands. 

And then there's mercenaries. Mercs don't give a rat's @ss about rules or traditions or etiquette. Mercs only care about getting paid. They are loyal to themselves and the gold in their own pocket, and that's it! This makes mercs inherently chaotic. Sure, a bunch of mercs might form a raiding party to pull off a bigger job, but they never become a Lawful structure. They remain a bunch of individuals loyal only to themselves, temporarily and perhaps reluctantly working together only because it increases their chances of getting paid more. Their only "leader" is the guy who everyone's afraid of. When a Boss hires a band of mercenaries to do their dirty work, that Boss is effectively trying to push a square peg through a round hole. That Boss may temporarily be able to use the mercenaries to burn villages, or murder enemies, or scare off competitors, but that Boss will never truly be in command of those mercs. So the Boss now has TWO problems! One - the mercenaries will always want more. You can't simply say, "Okay, the job is done, here's your gold, please go away." The mercenaries know you have more gold, and they want it. So even if they go away today, you have already given them an inside look at how much wealth you have and how your hierarchy is organized. Now they now how to hit you and how to hurt you. They will always have that leverage over you. And Two - the people in your Lawful structure who have been toiling away for a monthly wage for decades will see you handing all that gold to a bunch of filthy mercenaries, and that will breed resentment within the ranks. Your workers will begin to question YOUR loyalty to the structure. So the Boss now faces threats from the outside AND from the inside! 

Organizations are remarkably resilient, until they aren't. Like a mountain, or a building, or a bridge, the organization can withstand a lot of strain and erosion for many many years. But there will inevitably come a moment, a split second, and that's all the time it takes for strain to turn into collapse. To someone on the outside it will appear as a sudden anomaly. A structure that has stood strong for ages comes crashing down in an instant! How is that even possible? But to someone on the inside, someone who has witnessed the years of strain and erosion, they will be surprised that it lasted as long as it did. Organizations don't fall because of one big hit, but rather they die a death from a thousand cuts. Every time someone in the structure becomes disillusioned, every time someone high in the structure treats someone lower with contempt, every time the Boss disrespects the traditions of the structure (like by hiring mercenaries!), every time the local townsfolk see the structure treating them like servants, every time the structure takes more than it gives. All those little moments of annoyance and indignation will live in people's hearts. And when enough people feel that way they will join together, and annoyance becomes anger, righteous indignation becomes violent revolution. 

So how is that translated into game terms? The party is level 7 and they've found a clue or two about who burned the village. A level 7 party would have no problem finding and killing a band of 40 or 50 mercenaries. But that's too simple. And it doesn't stop the Boss. So you mentioned that the Boss wants chaos to break out (which makes her more of a demon than a devil, btw). Why? Why would the Boss go through the effort of climbing the ladder and achieving a position of leadership in a hierarchical structure just to burn it all down? Once you answer that, the rest of the story writes itself. 

Oh! How about this! (and this is just an idea, feel free to use or ignore)...  The Boss only looks human! The Boss is actually a shapeshifted Arcanaloth!   https://www.dndbeyond.com/monsters/5194899-arcanaloth     The Arcanaloth knew the organization had a treasure trove of books and secrets, and it figured if it got ahold of those books and secrets it would give it considerable power. So the Arcanaloth shapeshifted into human form, used its considerable intelligence, charisma, and magical abilities to assume control of the structure, and once it has what it came for it will burn everything down around it!  So maybe the local guard and some of the higher-up people in the organization have begun to notice "irregularities" in the Boss's behavior. Maybe the party could encounter some of those folks in a tavern and learn about this. Maybe people inside the organization are getting suspicious. And when the Boss hired the mercenaries, maybe she also gave the mercenaries a little bit of Evil Power. So as the party gets closer to the Boss's city, they'll notice a gloom hanging over the region, people are suspicious and paranoid, and groups of 4 or 5 mercenaries at a time are prowling around wreaking havoc and keeping the townsfolk too scared to fight back. Maybe the Evil Power has given all the mercs darkvision, and they do a little extra necrotic damage, and make their eyes glow red or some spooky stuff like that. So the party will easily know who the bad guys are, and there will be a series of skirmishes against mercenaries. But eventually the party will get close enough to the Boss's headquarters, and enough of the mercenaries will have been killed already, that the Boss tries to use their position of authority to command all remaining resources to defend her - that means local guards, underlings, everyone. This is the Final Showdown. The party will have a chance to use Persuasion to convince the guards that the Boss is not who/what she claims to be. There will be a moment during the standoff where no one is sure which side the underlings will be loyal to. But the Boss will recognize that the organization is crumbling, so she/it will decide to go out with a bang. The Arcanaloth will reveal itself (and of course it will have another dozen evil mercs nearby) and it will simply try to do as much damage as possible before Teleporting away. This will give the party a good fight, and it will give the party's caster a chance to shine, as the caster desperately tries to Counterspell the Boss's Teleport. Ultimately, the party will probably kill the Boss, but at great cost - a lot of guards and underlings get dropped in the fight, and the city no longer has any leadership. So maybe the remaining guards reward the players by granting them leadership positions to help rebuild the city. Or maybe the guards rally around their Just and Wise commander and he assumes control. And of course he asks the party for their help in rebuilding the city and clearing out any remaining evil influence, and gives the party plots of land and other reward for their efforts. 

I suppose that could all be done within one Tier. The party travels to the Boss's city, talks to the locals, learns what's going on, fights a few mercenary patrols, and begins to formulate a plan. That gives them advancement to Level 8. Then they might use stealth to infiltrate the city and scout it out, but somehow they will make contact with someone of authority in the local guards and convince them that the Boss is evil and has to go. And maybe they even get a bunch of the local townsfolk to assist with the effort. They have a skirmish or two in the city, kill a few more mercs, and learn the true nature of the Arcanaloth Boss (maybe by casting Speak With Dead on a merc they killed). This gives them advancement to Level 9. And that's when the final showdown with the Boss happens. A Level 9 party of 4 to 6 members against a CR 12 Arcanaloth with maybe two dozen low level thugs. That's a good messy fight. 

Sorry if I rambled. Hope this helps.