In my home-brew campaign setting of Cotterell, the barrier between it and the other planes and dimensions is severely weakened due to a magical cataclysm. In addition to the explosive influx of magic causing a sharp increase in aberrant monsters, there are now weak points scattered across the land which allow easier travel between the planes. Not all of these spontaneous gates lead somewhere dangerous or threatening. But enough of them do to cause most folk to stear well clear of them, when discovered.
Arguably worse than these are the places where the veil between worlds has worn thin, but not yet thin enough to allow a breach. All it will take, though, is something to push the veil to the breaking point to cause the worn spot to rupture. Powerful magic used indiscriminately, perhaps even a build-up of negative energy or emotion, and that point will rupture into another opening between the planes, often with horrible consequence. Unfortunately these weakened points are hard to detect. At least from our side of the barrier.
On the other side, the demons watch.
For whatever reason these weak points, all but invisible in the lands of Cotterell, are not only visible but stand out as dark flares across the abyssal realms. Demons are drawn to them, searching for ways to weaken them enough to gain entry to the world. If you pictured a demon or demons on the other side of a weak point, pacing back and forth like a tiger in a cage, you wouldn’t be far wrong. Luckily, most of a demon’s powers cannot cross into our world at these points, and so the demon’s influence has to be more subtle.
How much the demon can influence things on the other side depends on the size of the weak point itself. If the point is small, about the size of a dinner plate, the demon may only be able to affect an area about the size of a small home. Larger weak points, however, allow a demon or demons to influence things over larger areas; pity the village unknowingly settled in a lake-sized weak point.
So what can demons actually do through these weak points? First, their mere presence at the weak point for prolonged periods affects the emotions of whoever is within range on the other side. This can manifest in many ways, but is generally seen as an overall sense of hopelessness. Negative emotions become heightened and positive ones are diminished. Staying in the area for longer than a day will cause a character or NPC to take disadvantage on their Wisdom checks and saving throws. Second, while the demons cannot directly speak to a person when they are awake, they can whisper to that person’s subconscious while they sleep. These dark whispers make promises and try to push the recipient to fell deeds, all ultimately designed to weaken the veil enough to allow the demon to cross over.
It does take longer for the effects of this to take hold, however. A demon must whisper to a particular person for at least a week, continuously, before they can gain any influence at all. After a week of constant nightly dark whisperings, the person must make a DC 12 Wisdom saving throw (with disadvantage, of course) to avoid the demonic influence. If they succeed the influence is staved off temporarily. On subsequent nights, however, the subject must make more Wisdom saving throws, with the DC increasing by one each evening. Eventually the subject fails, and the demon is able to take hold in the victim’s mind. Now the demon can begin influencing the victim’s waking actions as well. These actions will vary, but generally work their bloody way toward the goal of opening the veil between worlds. The victim rarely survives this result.
That’s one example of demonic influence in my campaign. How do you handle it in yours? Let me know in the comments.