toucanbuzz
No rule is inviolate
I'm a fan of hardcore darkness when it comes to the demon lords. A couple dead plants and your dog barks at shadows is cliché and not liable to scare players.
An abyssal breach could be a sphere, perhaps miles in size where, in the outer area, all color has faded from all life therein, representing the pseudo-reality of blended realms. Within a certain radius, the minds of sentient creatures are plagued with the influence of a demon lord. One begins to question their sanity and ethics as their mind runs free into the most depraved thoughts, whether the being wants to see them or not. And, sickly, some part of them wants this to happen. Mechanically, you could use the madness tables from Out of the Abyss, when one sees a demon lord, for suggestions on how to fine tune this to the individual character. If you don't have the book, simply take the nature of the demon lord and make up stuff that fits. At this point, no saves, just flavor.
This type of environment is ambrosia to demons. They drink this up, revel in it. They take greater satisfaction in breaking mortals than they do in simply killing them. Further in demons soak up the despair and confusion of mortals, fueling and feeding it. Promises are made that will never be kept, both auditory and in the mind. Deals are offered with horrific cost, but somehow they make sense. Hallucinations blur the line between real and not real. At this point, you may want to bring in the DMG's madness save tables.
At the breach itself lies the essence of the level of the Abyss to which the tear ties. At this juncture, some shade of the demon lord is present to tear apart sanity itself. At this point, a DMG long-term madness save might be appropriate, and you'll want some feature of the level of the Abyss to which it ties to blend into this realm (could be nearly anything, but it should warp reality in a major way to provide a terrain or environmental challenge...I wouldn't take away life support as the Prime Material is still supreme here but most anything else goes).
This is also the vulnerable point where those with proper lore might know a ritual to seal the breach. The deals are offered again, but now with force. A player can easily affirm, as they sit at a game table snacking on Doritos, that "my character refuses anything offered to him. Die foul beasts!" But, you may toss another layer of madness at them in the form of a save. It's not simply about brave words. Only the most resolute even have a chance here. And once the demons sense that the mortal will no longer feed their appetites, they act in the other way they know best, and that's with violence. Because these demons are being forced to guard a breach instead of their normal fare (such as enticing mortals, fighting in the Blood War for status, etc.), they're likely compelled by a greater general demon. Stop the general, the lesser demons might lose their taste for guard duty.
An abyssal breach could be a sphere, perhaps miles in size where, in the outer area, all color has faded from all life therein, representing the pseudo-reality of blended realms. Within a certain radius, the minds of sentient creatures are plagued with the influence of a demon lord. One begins to question their sanity and ethics as their mind runs free into the most depraved thoughts, whether the being wants to see them or not. And, sickly, some part of them wants this to happen. Mechanically, you could use the madness tables from Out of the Abyss, when one sees a demon lord, for suggestions on how to fine tune this to the individual character. If you don't have the book, simply take the nature of the demon lord and make up stuff that fits. At this point, no saves, just flavor.
This type of environment is ambrosia to demons. They drink this up, revel in it. They take greater satisfaction in breaking mortals than they do in simply killing them. Further in demons soak up the despair and confusion of mortals, fueling and feeding it. Promises are made that will never be kept, both auditory and in the mind. Deals are offered with horrific cost, but somehow they make sense. Hallucinations blur the line between real and not real. At this point, you may want to bring in the DMG's madness save tables.
At the breach itself lies the essence of the level of the Abyss to which the tear ties. At this juncture, some shade of the demon lord is present to tear apart sanity itself. At this point, a DMG long-term madness save might be appropriate, and you'll want some feature of the level of the Abyss to which it ties to blend into this realm (could be nearly anything, but it should warp reality in a major way to provide a terrain or environmental challenge...I wouldn't take away life support as the Prime Material is still supreme here but most anything else goes).
This is also the vulnerable point where those with proper lore might know a ritual to seal the breach. The deals are offered again, but now with force. A player can easily affirm, as they sit at a game table snacking on Doritos, that "my character refuses anything offered to him. Die foul beasts!" But, you may toss another layer of madness at them in the form of a save. It's not simply about brave words. Only the most resolute even have a chance here. And once the demons sense that the mortal will no longer feed their appetites, they act in the other way they know best, and that's with violence. Because these demons are being forced to guard a breach instead of their normal fare (such as enticing mortals, fighting in the Blood War for status, etc.), they're likely compelled by a greater general demon. Stop the general, the lesser demons might lose their taste for guard duty.