There are a number of considerations with regards to this "Nightlands" setting that need to be resolved before heading much farther in.
First off, how big are the safe havens? Is Lumina a major city? If so, how does it sustain itself? Dark areas aren't really much for farming, and unless the haven is just outright huge, traditional farming won't work either. Secondly, what about trade? Does it exist? Are the nightlands so dangerous that even heavily armed trade caravans can't make it? If that's the case, do the citizens of Lumina even know there are other havens? For that matter, how dangerous ARE the nightlands? Is it possible to have a civilized area outside of a haven? If that's possible, why have people stayed inside Lumina for so long? If it's been long enough that the history of the world has been largely lost and forgotten, who would have any viable adventure leads or quests to send the party on? I'm not really going for a West Marches sort of thing, so there needs to be someone with some knowledge of the outside. What is protecting Lumina exactly? What about the other havens?
OK, so that's a bit much to cover in one post. We'll start with a few basic facts about the nightlands and go from there.
I see the nightlands as dangerous to the average commoner, even nearly suicidal for one, but for the superheroish PCs from 4e, the nightlands approach survivability. This means that though the average Lumina rank-and-file won't step outside, the city can make brief expeditions outside the haven if they have a tolerably sized military squad or hire some PC-level mercenaries. This has an effect of also allowing limited trade, but only of high-level items. There's no profit in running food from place to place, because of the protection required for the caravan, but moving magic items and the like might be worthwhile, especially given the (ridiculous) merchant markup in 4e.
So the citizens of Lumina stay in the haven because it's too dangerous outside. What makes it dangerous?
Well, there's the darkness first and foremost. Darkness has always equated to danger in human history and there's no reason to assume that would be all that different in a fantasy realm. Of course, darkvision exists for non-humans. As far as real danger goes, there are monsters. Since the area outside of havens is both dark as well as cursed, there's a reason to have evil/aberrant monsters all over. Mind you, that doesn't mean that the first-level party in their virgin adventure is going to run into Beholder Eye-Tyrants camping outside the city gates, but there should be a persistent sense of danger and the unknown in the nightlands.
I think the prevailing monster type should be undead and dire animals. Now, I haven't read 4e's monster manual all that much. I've heard that the designers sort of shied away from templates and whatnot, so making your own variant monsters (zombie wolf, for instance) is harder than before. Well screw it. Before each session, edit a few monsters from the manual to make them a starving/dying/undeadish variant. Instead of a dire wolf being a wolf that is bigger and vaguely spiky, have it missing the flesh from half of it's face, with a festering wound on it's left flank, growling low while staring at the party from it's cloudy, dead eyes. The creature abilities are the same, excluding perhaps a vulnerability to holy/radiant damage, but there's a bit of a ruined, dark flavor to the thing.
Can there be civilization outside the havens? I say yes, but only if you can use 'civilization' loosely. For that matter, we can go as far as this: Goblinoids and Orcs are humanoids that were outside the havens when the darkness fell. The ones that were able to survive were slowly changed by the cursed land, becoming orcs, goblins, bugbears, what-have-you. Note that they have no reverence for humanity, and they have no real knowledge of their origins. This information might be hard to discover for the PCs, if it were even possible. Anyway, Orcs and Gobbos are still basically evil and savage. Their skin is darker green than MM would show, as an adaptation to their environment. It's been known that the harder life is in a place, the slower technology and society develops. If there is no leisure time due to constant danger and hunger, then you rarely progress beyond hunter-gatherer groups. So that's how the orcs and goblins are. Some settlements, but mostly neolithic technology. They do have tolerable weapons, since they need them to survive. Perhaps they looted them from old ruins, and treat good weapons with a reverence that civilized peoples reserve for the gods.
So are there other havens than Lumina? Yes. But I will leave those for later. I think Lumina and the surrounding environs are more than sufficient for getting through heroic tier.
Perhaps more on this setting later today.