I am currently writing a campaign which I hope to start in the next few weeks. My current concept is about an NPC who has stumbled across a magical artifact that allows him to bring back whoever he wants from the dead but it goes badly wrong. With the premise of "Everything has a price". 

I am struggling with fluffing the narrative out from a story purpose. Im thinking ghost npcs that require the PCs to help them be released from the resurrection. Swarms of zombies, ghouls etc for the party to fight. With the NPC, a now Necromancer, as the final boss. The problem is so far that sounds really bland and definitely needa fluffing out.

Any help and ideas would be greatly appreciated!!

Maybe the NPCs he is raising are not undead? maybe he is switching their souls with demons. And slowly the city will be infiltrated by those possessed individuals.

Sounds like Pet Cemetery.  For me, I would blend it with TWD and have the risen dead seeking out final death.  Yes, they are moving in a horde as zombies or skeletons, but their souls are aware and intact in those rotting shells.  Deprived of the release of death, they seek it by throwing themselves at the Heroes, large campfires, or other dangerous things.  When the PCs encounter the undead make a point of highlighting non-optimal choices the undead make, almost like they *want* to be destroyed.  Skeletons can't do this, but zombies might be able to wheeze out a "Kill me!" faintly as they bite and claw at the PCs. 

A  caveat here that what I am about to describe is heavily influenced by a style of GM'ing, which may-or-may-not suit your personal tastes, so picture me saying "in my opinion a lot"

You're off to a good start: You have a basic Premise, you have a Theme you want to explore - everything has a price -  and you a rough concept of the Villain: the NPC who has found a resurrection artifact which they are trying to use ( although it's not working as they had hoped ).

However, A GM doesn't need to write the story. That's a collaboration between the GM and the Players - so you really can't write the Narrative.

What a GM does ( IMHO ) is set up a set of initial conditions, an initial conflict, and designs the NPCs & the Setting - and then those conditions evolve, along with whatever insane actions the Party tries, to create a series of events some call Narrative. An Adventure is the series of events that unfold because of some Conflict between what the NPCs are trying to do, and what the Party wants to happen.

So - first you need a Conflict, which means you need (a) Villain(s), and they need Goals - ideally a complex goal, where they want to accomplish X, so they first need to do A, then B, then C ... ultimately getting to X. This gives the Party multiple chances to thwart the Villain's plans. In your case, what does the Villain want to accomplish with this new power? How can you set it up so that they can't accomplish their Goal right away without the Party having a chance to intervene? 

If they have Goals, then they need Motives: why are they trying to do what they are trying to do? Is their motive power, in which case they're trying to raise an army of undead soldiers? Are they motivated by grief and loss, so they are trying to revive (a) lost family member(s)? Are they ideologically opposed to the Gods condemning mortals to die, and so want to upset the natural order of things? 

The most interesting Villains are the ones whose Motives are sympathetic ( they want to resurrect their child ), but their Methods and Goals are reprehensible ( experimentation on "less important" people to perfect the process before they use it on their child ), which can give the Players some emotional conflict about even opposing the Villain's plans.

Now you need to flesh out the Villain more: what resources do they have on hand: material, people, and social capital ( reputation to leverage, favors owed, etc ); what is their personality: what are their "go to" tactics, what will they do in an emergency, what will they never do; what do they know & believe ( remember the Villain can hold mistaken beliefs ) about the situation they find themselves in?

Remember - you can have more than two factions in the conflict. It's not always just the Villain ( plus minions ) and the Party against each other. It could be "The Necromancer", "The Party", "The Temple", and "The Thieves Guild" involved - all with their own set of motives and their own agenda. Not all of them have to oppose the Party - some can be allies, some can just be interested parties themselves. The adventure can get interesting as the Villain pursues sub-goals ( A,B, and C ) and alliances between the groups change and shift as the Villain's goals shift: maybe the Party and the Temple both oppose goal A, while the Thieves Guild supports it - but when if comes to Goal B, the Temple and the Guild are in agreement with each other but are opposed by the Party.

Now you bait the adventure hook: How will the Party find out about the Villain(s) plans? What motivation will the Party have to oppose the Villain ( hired to stop them, the Villain is targeting their hometown, etc. ). The Party needs goals of their own. 

Then you need "ending conditions": under what circumstances does the Conflict end because someone has won? Under what conditions does a Group withdraw from the Conflict?

With that, you could play out the Adventure. You figure out what the all the various Groups ( Party, Villain, and Others ) want, believe, and can do; You figure out what they will try and do next; You figure out where those actions will come into Conflict; you resolve those Conflicts off camera when they're between NPC groups, and you play them out as Encounters when the Party is involved; repeat until either all of the Groups has reached an "ending condition" and the central conflict in the Adventure is over, or until all the Groups have stopped participating.

There are some extra things you can do to up your game however. 

You can flowchart out the possible unfolding of events as you see it: the Villain will try and do this, if they succeed, then they'll go off and do that, but if thwarted will try this other thing. Each "step" in the flowchart should be a point of conflict between the Groups, and the branches will be the possible outcomes. Where those points of conflict involve the Party, that's an Encounter. Put some work into fleshing out and designing those Encounters; designed Encounters typically are better then things we improvise. You don't need to plot out all possible paths through the flowchart until you know all possible ends to the Adventure ( that could be huge if you have multiple Groups ), but so long are you're projecting out far enough that you can keep ahead of the Party, you're doing OK. However, it is important that once you've sketched out those Encounters that you ignore the flowchart. It is not a series of paths that you are supposed to nudge the Party down ( and you will be tempted! ), it's a predictive tool only. Let the Party do whatever they want. If they veer off the flowchart ( and they will! ), let them,  improvise, remember what they did, and try and predict better next time.

You can have a stack of Plot Twists in your back pocket: a new Group just entered the conflict, something or someone was not as you thought it was, etc.

Related to plot twists are external Events - such as storms, festivals, wars, etc. - which aren't caused by any Group involved in the conflict, but affect the ability of one or more of the Groups to act. External events can be a surprise ( sudden storms ), or known about ahead of time to give a sense of urgency to some of the Groups - the reinforcements arrive in 4 days, which will drastically strengthen the Mercenaries' position, so we need to resolve this before then!

Plot Twists, and Events are great tools to help control Pacing: when things start to bog down, throw another one of these on the conflict to keep things interesting - but try not to be too obvious or heavy handed doing so.

With all that in play, the Narrative will write itself. You don't need to write the plot from beginning-to-end; you just need to know the Setting, the Conflict, the Groups involved, and Ending Conditions ( that's all, right? :p ).  From the initial setting y ou can role-play out the actions of those Groups, in response to the actions of the Party & each other ( or Plot Twist, or Events ), until you hit the end.

It's a lot - but I hope that gives you some ideas on how to set up an interesting Adventure seed, rather than trying to come up with the whole Narrative from beginning-to-end. That will come when the Players collide with the Adventure setup you've created ;)

Best of luck!

Amazing post Vex.  It almost like you have done this a time or two.

Thanks :) I totally missed addressing setting Tone and the Player experience - but your post captured that in spades :)

Your setting pitch sounds terrifying! Love it :D

As an idea for unexpected consequences more and more of the alive Townfolk guards etc. start to become hostile so they start reclusing themselves and before they know it you’ve made them the bad guy.