I haven’t thought about this in a while, but what are some ways you all like to handle total darkness – say, if there’s a creature that makes a room supernaturally dark. I recall an old method involving an extra percent miss chance, but curious to hear alternate methods?
And similar question for handling invisible enemies, and making those encounters interesting, challenging, and still fair? 
What are your rules for blindness? In general having some idea about conditions - something players can review or comprehend easily is helpful: blindness, exhaustion, pain, blood loss, drunkenness, magical old age, mite infestation – all are good to have rules for because they provide a bank of effects you can draw on for analogous situations.
Generally in the sorts of game I run darkness/blindness is a -4 to all rolls except listening, a similar penalty to armor class, and anything that can see in the dark gets a surprise round if (or when - the hungry denizens of the underworld always attack obvious weakness) they attack. Magical darkness, the kind that eats normal light, is a great trap/obstacle - especially if there are traps within: pits, animated nooses hanging from the ceiling, scything blades, zombies, etc.
Invisible enemies are slightly different, I tend to expect players to be able to quickly figure out where they are by throwing flour or chalk dust in a cloud. Even a spray of wine, water or oil works I guess. The danger is in the initial surprise attack (assuming the enemy chooses to do so) - the usual “backstab” mechanics are lethally horrific and applying the to players makes for a very, very nasty encounter, especially if the invisible spider, ghost Lizzy Borden, or translucent knife armed goblin etc is smart enough to stab and step away, following to stab again more or less at random.
I have done without detailed rules on all kinds of things for a long time. I think I remember blindness being -4 in whatever version of DnD I used to play, but in most of the games I run nowadays, it isn’t accounted for.
It’s worth asking, I think, in case there are folks who’ve created procedures that attack an old problem from another angle. For example (not about blindness, but other stuff I used to ignore or take for granted), innovations like “usage dice” and level-based “ongoing damage” in The Black Hack, for poison and fire. Or, say, ability score damage as a way of pacing losing consciousness, like in @Jeremy_Strandberg’s “Drowning and Falling” and, to a lesser extent, ability damage in Into the Odd.
With regards to darkness and invisibility, I suppose I was looking for some clever thinking around zone-based movement or zone-based conditions (like Situation Aspects in FATE Core) maybe. As usual, I’m hoping to hear something new that may or may not exist. Or something old-but-novel that I might have missed in the past.
@GusL - I definitely appreciate your perspective, as always! The balancing and sudden lethality are definitely things that have made me wary in the past.
The other part is that I’m creating some monsters in my head that riff on the anti-magic cone of the classic Beholder. I’m thinking to myself, “Would it be interesting to have a monster emit magic Darkness out of a giant eyeball in its chest?” 
Like, the usual thoughts at 2:00 AM.
FATE does have the bronze rule, that anything can be a character. As such the darkness itself, or the dark room itself can be a character. As can blindness. The dark room could be a character with certain skills or approaches, like the ideas of @GusL who talked about traps in the room.
As such the room would be something like:
Aspect: Dark room
Aspect: Filled with traps
Skills: Obscuring Darkness (4), Silent Animated Skeletons (3), Pit Trap (3), Hanging Nooses Hungry for a Neck (2), Scything Blades (2), Zombies (2)
Of course, they don’t need to all be there, and NPC’s don’t need to hold to the same skill pyramid as PC’s do. They can even use approaches, at which point I’d go for Obscuring Darkness (4), Hidden Traps (3) and Hidden Monsters (3)
To translate it to a more OSR friendly conversion you can give skills or attributes or an HP track or something to the room. Or give it attacks that require saves using the system of choice or something?
Running out of Light in some of my Dungeons is sort of a Fail State unfortunately. Information gathering and navigation are crucial in these places, and these kinds of exploration speeds get even more glacially slow without that sensory input to rely upon. This in turn, creates a hampered ability to perceive Traps/Hazards as well as triggering more Encounter Checks which increases the likelihood of a bad situation: The Monsters can see just fine, the Dungeon takes care of Her children.
With access to Infravision, play can proceed at a somewhat slightly faster pace. Navigation is still hampered when the Map is no longer useable, but at least some members of the party can see to negotiate the immediate surrounds. It’s not ideal of course (shades of ShadowRun Decker problem sometimes), but again: The goal is usually shifts to re-obtain a Light Source ASAP.
It’s a desperate situation, but most of the tension happens before this point, and I’m not stingy with telegraphing this, especially to inexperienced players. It has led to making a bad situation much worse (players have even gone so far as to light their Map on fire to have a few flickering moments to try and find something a little more durable to hold flame, a table leg, a dry sack, etc.). But getting “stuck” in a Dungeon can lead to an unsavory end, up to and including rolling on my “So You Didn’t Make It Out of The Dungeon” table. 
Magical Darkness might even the odds a bit. You’ll need to determine if this is impacted by Infravision or if Monsters are able to perceive through it. Continual Darkness specifically hampers Infravision, and I generally rule that it has a similar effect on the Monsters ability to “see” in Dungeons.
Invisibility is of course is more akin to a puzzle than something terminal. Player Skill becomes crucial here for figuring out a way to neutralize this advantage the Monster might have over them. Invisibility confers a high likelihood of Surprise (5-in-6) and a Attacks and AC improve by anywhere from 4 to 6. This stacks the deck in the Invisible Creatures favor significantly, but Encounters in these conditions are significantly better than Blindness (where targeting an attack is simply impossible under most circumstances). They can still be somewhat challenging to run. I know I had quite a time coming up with my 100 Encounter Activities for the Invisible Stalker.
I have been known to incorporate the Super-Hero (8 HD Fighter) see invisible creatures feature from Chainmail and OD&D on occasion, but this ability vanished almost immediately after the LBB. Like Magic Swords being Fighters Only I do miss it as a paying field leveler.
What a great question! I ran into this the other day reading Dark Heresy 1e, which has specific rules for becoming invisible, but no rules on what it means to be invisible.
I find D&D 5e’s solution to be one of the simplest, if not the most elegant: you are assumed to be able to determine location by sound, such that you can attack at disadvantage. However, character cannot be seen so doesn’t trigger a bunch of “one character you can see” conditions for spells and the like. It’s easy, it’s conceptual. It’s baller. You can break it down into skill checks and passive perception with Hiding mechanics. There’s a real simplicity for it. Of course, that’s also a massive problem because there’s no granularity (for example if you already have disadvantage on your roll, the invisibility means nothing). It also doesn’t “feel” like invisibility in a game like that. It also responds INCREDIBLY poorly to the tactical minis part of the game. All of that is an honest mess. But it’s entry level, so its fine.
Negative modifiers, stepping back outcomes, extra rolls as you pointed out. It’s truly interesting. This kind of thing that tactical wargames work around with fog-of-war and especially smoke (WW2 settings). There are entire armies in Flames of War built around smoke. The benefit of smoke there is because it’s a thin line of line-of-sight-blocking obstacles, the point is to force movement. The movement then reduces tank rate-of-fire and provides manoeuvring opportunity.
In the kind of FKR, Kriegspiel methodology, darkness/blindness/smoke are all both strategic and tactical limiters. Strategically, the GM passes less information on to the players (in a dungeon crawl, imagine this as taking damage or making dex rolls without really knowing the context behind it). Tactically, it reduces the impact of the crews involved: A slower advancing speed, no long-range shots on a charging enemy. And I think both avenues are valuable.
The Interesting, Challenging, Fair triangle is a cool question here because usually interest comes from a question left unanswered: roll 1d6, take that much damage. “what from?” Great question, your arm feels hot and wet, but why, you don’t know.
Challenge comes from providing an adversary with axes of attack and defence, which means you need a way to nullify the darkness’ effect. Perhaps that’s with a spell of light, or perhaps it’s by throwing a bomb forward and just hoping you hit whatever is threatening you over there. This is where the “challenge” of locating by sound can be really interesting: Rolling perception rolls for the group as you attempt to narrow down a location for return fire. I imagine this in a military/spec ops setting as a skill challenge to overcome the darkness (cf @TheBeardedBelgian and their comments about FATE, in which negatively charge aspects/characters exist to be both Overcome and overcome). HP-for-Darkness is a good way to think about it too, engaging with the extant mechanics to overcome it.
Fair depends on the scope of your game and the economies in play. In FATE, are there ways for players to Overcome, and spend and receive fate points? In an OSR game, are there tactics in which they can avoid or reduce the impact of darkness? In D&D, have you considered the darkness within the challenge rating of the encounter (some monsters that can produce darkness or have their own invisibility have this inherent, some do not).
Encounter design is a really interesting pattern of planning, and I don’t think it’s as simple as “is darkness interesting”. In a kobold v adventurers slug-fest, or a runners v prisec shootout, darkness with extra miss rolls just extends the whole combat symmetrically in a very boring way. But if the challenge is “do you assassinate the high value target”, a Prisec team can cut the lights to create an opportunity to evac. If the challenge is “do you escape uncaptured”, adventurers can cast Darkness to give themselves to provide a clean out with the conflict that they also can’t see.
In wargames, smoke encounter design is about providing counterplay tactically (as I said, in FoW, smoke is a thin line that can be avoided or waited out), and building the cost into the unit strategically (as both a point cost for balancing, and an opportunity cost for use).
I think as a standard, consider how it makes DIFFERENT things difficult, as opposed to how it makes the regular challenges MORE DIFFICULT. ie it’s not about “can you beat the armour class of this monster”, it’s about “can you move through the difficult and dangerous room, in order to make that one-hit-kill at the end”.
Also “A Blind Legend” on steam. It’s an adventuring story where directional hearing is used to direct you.
Hi there, this is all interesting and very thoughtful. I shall ponder it more. Thanks!
It’s worth trying to experience full darkness (underground) to see what you think the rules should be. I’ve done this a few times in caves, and once in an underground restaurant in Cologne (“Unsichtbar”, a wonderful pun of a name) which is in compete darkness, and all the waiters are blind.
True darkness is so much more of a handicap that you might expect it to be.
Seconding this recommendation. When I was young, we went to some famous caves on a family vacation and there was a point in the tour where deep underground, they switched off all light sources for a few minutes and we were able to experience that deeper darkness.
It’s really amazing just how disorienting it can be (and this was standing still), after a few moments, even the slightest sounds seem amplified but without a visual frame of reference it’s difficult to place sources. It’s a palpable and oppressive feeling, it seemed like I could almost hear the blood rushing in my head and steadily increasing heartbeat.