I assume that the game world is a mix of what you appreciate taken from book you read, movies/tv series you watched etc...

Study a collection of such material to see what has be done, and then take inspiration from these.

Coming to my mind just after reading your post and flowing :

- how was the artefact discovered ? who studied and established what it is and it's purpose ?

- how did organisations heard of this artefact, how did they struggle to get hold of it ?

- how would the demonic invasion be decided, and by who ? Is it the handling of the artifact that attract the attention of the demon, or an occult cult that invoke it ? (hey, did I mention occult cult ? what would be this cult ? what's it's master demon, beliefs, rituals, etc ?)

And so on...

OK, I'll play. How about:

Everybody is a clone.

This reality is not real, but someone/thing's dream/computer program/storybook.

The demons are the "victims" of the human's invasive colonization efforts millennia ago.

A malevolent alien species is simultaneously attacking the earth from space.

The war is melting the polar ice caps causing another worldwide flood.

A supervirus turns humans into zombies and affects the demons even worse.

Magic suddenly returns to earth giving humans untold powers to use against the invaders and each other.

Secret super-agencies develop an army of robots to fight, but are hit by a computer virus making them zombie-robots.

Mad scientist develops earth-core bomb to blow up earth since he is convinced we can't win this fight. Succeeds in leveling the Alps.

Another scientist develops time-travel device, but causes this earth to go back 100 years so it's in synchronous orbit exactly opposite the other earth so it can't see this one.

Ghosts get the ability to materialize and fight as they see fit.

Legendary creatures/old gods wake up from their assorted lairs and prisons to do as they want.

Seriously. Pick your favorite movie/book/RPG/fairytale/Myth and run with it. Creativity is how you go about it. Somewhere, someone listed all the possible plot devices. Look at that show "Defiance." Nothing new there, and it looks like it's doing well in the ratings. I've already grown bored with it, but there's not much else on TV these days. Let me know if you need me to list some more tried-n-true meta-plots. I'm only just getting started!

I have to ask, why are you choosing to create an RPG with so little of the story created? What would you consider your strengths as a game designer to be? RPGs, as you probably know, tend to be very story oriented, so I'm wondering why you're choosing this genre.

With that said, it sounds like you have the basic premise of the world setup with that initial description. I suggest focusing more on your characters at this point. Where does your hero fall into this story? Do you have any villains? Supporting characters? Try to write out an initial cast with some varying personalities. After playing around with some ideas, you may find some subplots created naturally for your world. I hope this helps.