This question is somewhat based on the topic Silly Campaign Idea: World of Light
Basically my main worry about GMing such a game (aside from my being very inexperienced as a GM and if I ran it it would be my first game) is the fact that within the circumstances no matter whether you go for one (Canons) or the other (unique PC designs), there would be a relative lack of NPC characters due to the world they live in basically being erased until all people except for a number of heroes were turned into Spirits. Hence asking the topic question, is it possible to do a story based game without NPC characters? and if not how can I introduce NPCs into this?
Well, first off, NPCs are everyone (and thing) that isn't a PC, so yeah it is pretty much impossible to have a game without NPCs.
I think you may mean friendly NPCs, which are generally useful but not absolutely necessary to a campaign.
The big issue, it seems to me, based on that other thread, is that your idea is basically a setting for endless fighting. From what I gather, all that will happen in your campaign is one fight after another. Nothing else. No story, no real plot.
If that is true, then you probably don't need any friendly NPCs, although I don't see it being a game that will last more than a few sessions at most.
If their is a plot, most likely you will need friendly NPCs to help advance the story line and give the PCs needed information and direction. Most likely the needs of the story will inform where the NPCs came from and what their motivations are along with how they will relate to the PCs, but without understanding your story I couldn't give any further advise.
Some campaigns have a vast plethora of NPCs which can be really overwhelming for newer GMs, but i think the antithesis of No NPCs will make it really overwhelming for your players. Only being able to role-play with one another will quickly exhaust them. There is only so much backstory one can reveal or character development one can explore with such few characters to interact with. This could turn your table into sessions of just extended combats and puzzles.
Could you give the party some sort of power (perhaps limited by uses per day and modified by skill checks) that allows them to like peer into the spirit realm for interactions? I remember something like that from a video game (maybe Final Fantasy 10) or something. Maybe your PCs could sense the spiritual presence of a certain place and kind of tap into some sort of spiritual flashback to see the who, when, what that was present before the erasing of the world.
Maybe they can't directly interact with these spirits, but they can observe exchanges or occurrences and pick up on these as clues to advance plot narrative.
It's easy to work in a non-npc story telling narrative through found notes and such, without it being entirely an 'endless combat'. If you wanted to work in a little inconsequential, non-dialogue (more monologue-ish) story prompts, psychic or emotional impressions of what would have been a friendly npc depicting a step forward in the narrative could work.
One thing though is so it doesn't just become 'I rolled a 40 on my perception, wheres the hidden plot point', vary the way you use items and impressions; let's say players find a letter to where the towns [former] resistance is, but it's heavily coded and parts are written in magic, requiring both Read Magic, possibly an understanding of another language, and a high-ish linguistics check, or the cypher (which could lead to another quest).
To draw a parallel (though not a good one to say the least; and kind of off topic on Pathfinder as a whole), Fallout 76 is experimenting with absolutely no human (or full personality) npcs. The entirety of the story is told through 'found documentation' and environmental storytelling.
To be honest, if you could pull off a campaign like this well enough, it will also reward people for picking more skill intensive builds, such as investigative types and trackers. Unfortunately it diminishes the need for a 'party face' which means you would need to find a way to reward those who would like to invest in Diplomacy, Bluff or Intimidate for outside of combat use, or risk a bunch of dull, emotionless killers as a party. Possibly come up with debate puzzles? Although I find this a little hard to do with no 'friendly' npc to 'argue' against.
It seems to me that an easy way to add NPCs would be to literally add them as the PCs battle the effects of the evil force. Maybe the PCs free an area from the villain's influence and return a few people to normal. As they continue to fight they could free more people, who could provide them information, resources, or just additional goals. Some of them might become allies, some might need protection, etc. There's a lot you could explore there - consider all the ways people might react if they woke up to find themselves in a world overrun by this evil entity.
Sorry for kinda being quiet so long, and to somewhat answer things...
1: Admittedly I can see the potential issue with things getting to be just long strings of fights, even in the revised version (where rather than literally everything being destroyed there is a more standard villain that uses whatever shadow he has to basically forcibly take over the worlds so that there can still be living creatures).. Less because I feel like it would seriously be the fight-go-right style, but more because I imagine there being a lot of dungeon delving and boss fighting, going into evil strongholds in order to break darkness's control over a region one piece at a time) How can I make sure there is more of a balance?
2: WithoutHisFoot definitely has a point somewhat in regards to adding NPCs with time, though that matters less in the revised version where there still are living creatures, just ones thrown into a shadowy melancholy.
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