If you have a look at the Cthhulhu mythos, the 'gods' actually are rarely true all-powerful beings. They tend to simply be a lot larger than regular people, and bizarre and unknowable in aspect (a lot of CoC involves doing research to try and find ways of dealing with them, which is almost always 'possible' in a strictly technical sense even if it's far from easy). To be honest, even the gods in D&D tend to be fairly finite in their capabilities as they're usually made with PCs' murderous intentions in mind. I wouldn't have thought you'd have too much problem making them appropriately vulnerable even if you made them actual gods, overall.

You might like to have a look at Eternal Darkness: Sanity's Requiem one the Gamecube (or at least a plot summary/let's play of it), the cultist and godlike being parts of that are very well-done (especially the finale) and there's quite a neat spell system too.

I really should download an emulator for gamecube...

I think I'll go for a low-fantasy setting in a steampunk universe. Any idea for world building?

Ah, I approve about the role reversal. My PCs will play the evil and corrupting part.

Define world building? Holy texts, characters?

More like continents, major factions, major cities.

I'd keep things relatively simple, just one or two biomes per continent (the desert continent, the tundra continent, the forested continent). You could always just go for chains of islands, thereby every community is of a limited size and relatively isolated from its neighbours which is usually solid for horror/conspiracy settings.

Regarding factions, you'll have to decide if the cult activity is overt or covert: are the cultists already in charge of things, is the whole world already in the grip of their powers? Or are they all still underground? If the former, your factions are just basically your major cults (plus maybe a secret faction of hero-types who want to put things back to rights); if the latter, again I'd just go for simplicity and have a democratic state, a tyrannical empire, a religious hegemony, a military dictatorship (or mix and match).

Cities-wise, I think you'd be best served by only having one or two 'major' ones and keeping everything else on the small-town or even village level, if you're looking to foster some proper Lovecraftian atmosphere. Of course, all large cities should be built on a) a surprisingly extensive sewer network, b) a bunch of potent, mysterious old ruins or c) an extensive occult graveyard (or, again, all of the above).

I'm dubious that steampunk would really work, to be honest; one thing it rarely does well is low-fantasy and having things like Zeppelins, sky-pirates and Tesla-rifles rarely mixes well with anything dark and depraved. If you're really set on it, you should maybe have a look at the Dragonmech D&D setting by Goodman Games, that's a more medieval and crude portrayal of steampunk that works quite well in the context (plus there are evil dragon-tentacle-gods from the moon). Or play/watch the Thief series (I, II and III; the remake ruined the tone) which have some steampunky elements and a very dark and brooding atmosphere.

I will keep it a bit simpler and start with the basic world order first..

I would actually go for a pretty normal world with a single religion dominating most things.

Something like..The Church of the Pure Sun. Or..The Highest Order.

Here i would enforce that this Church is actually a very nice and pleasant place. there are of course intrigues between different factions within the church and some have a few other views than others..But its generally a genuine force of good.

I would introduce this because -if i understood correctly- your PC´s would bring corruption into this world. So it would be natural to have a powerful order trying to stop this.

I would probably make a Woman the Head of the church, kinda like in Dragon Age. You could call her "the First Ray" and make her an avatar of everything good in the world. Maybe even tie that to your backstory about the evil forces.

Finally i would separate our church into different branches like scholars, paladins, preachers, healers and the "Curators", who only have one task:

Clean up all opposition and destroy every evil they can find. Imagine them as Holy SWAT Teams. They have the authority to destroy entire cities if they find it necessary to keep the world in balance and on the light side.

With that you can on one hand escalate the fight for your fighters and on the other hand provide political webs for your social focused group members. Not to mention you can seduce nuns which is always something gleefully evil.

Edit: I just realized that this would work even better if you give the different factions names after the colors of light..Like the female leader is the white ray, the warriors are the red ray, the healers the blue ray, etc.