I'm DMing for two PCs, a half-orc and a half-elf. Both have Darkvision. I'd like to create a dungeon/combat session that uses darkness as a puzzle element, obfuscating their view and making them think about how to overcome it. However, since they have Darkvision they're never unable to see due to light levels. How can I introduce "darkness" that they actually won't be able to see through?
As the DM, you can introduce whatever you want, including darkness that Darkvision doesn't see through. However, if you're looking for options that already exist within the rules, there are plenty.
- The Darkness spell creates magical darkness that normal Darkvision does not see through.
- The Fog Cloud spell creates magical fog that heavily obscures an area.
- There are a bunch more "fog" or "cloud" type spells, but they have additional effects that you may not want (like a bunch of damage).
That said, all of those spells affect a fairly small area and last a fairly short time. You want to fill a dungeon and have it last an adventure? You want one of the most underused spells in the book. Guards and Wards. It does a bunch of stuff, but the one you care about is:
Fog fills all the warded corridors, making them heavily obscured.
It lasts for 24 hours, and can be made permanent with some work. It covers a huge area. It's really cool. It's almost as if it was made for this exact situation. (Because it was.)
The Hallow spell has an option to fill an area with magical darkness. RAW, it does not provide an impediment to Darkvision; however, it's a very small step outside of RAW for it to do so. This could be interesting, as you could have a temple that has patches of supernal darkness scattered throughout, giving opportunities for some very Zelda-esque puzzles.
Another similar option would be extraplanar spaces where magical darkness they cannot see through exists. The realm of the Night Lord Shargaas (see Volo's, under orcs) is a good example. The players could either be transported there, or the planes could be coterminous in a select location.
One common house rule is that Darkvision is actually Low-Light vision. This means that for darkvision to work correctly, there must be some amount of light.
However, in an environment where there is no light (ie deep underground in a dungeon), even creatures with Darkvision are unable to see.
I have used this house-rule with my group and it is generally accepted by the players as logical and fair. Using this system, if I want to make a dungeon or adventure element based on darkness, Darkvision does not become an issue.
Fog or smoke creates heavily obscured areas which dark vision and even devil's sight cannot see through.
Fog Cloud PHB. p243 Duration: Concentration, up to 1 hour
You create a 20-foot-radius sphere of fog centered on a point within range. The sphere spreads around corners, and its area is heavily obscured. It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it.
Perhaps you have a wizard or set of wizards maintaining a Fog Cloud on the party to keep them disorientated?
Guards and Wards PHB p.248,
Corridors. Fog fills all the warded corridors, making them heavily obscured. In addition, at each intersection or branching passage offering a choice of direction, there is a 50 percent chance that a creature other than you will believe it is going in the opposite direction from the one it chooses.
The "dungeon" has one of these cast upon them. You get lots of other things to play with too!
Eversmoking bottle DMG p.168
Smoke leaks from the lead-stoppered mouth of this brass bottle, which weighs 1 pound. When you use an action to remove the stopper, a cloud of thick smoke pours out in a 60-foot radius from the bottle. The cloud's area is heavily obscured. Each minute the bottle remains open and within the cloud, the radius increases by 10 feet until it reaches its maximum radius of 120 feet.
The cloud persists as long as the bottle is open.
There is one of these has been left open for some reason and it has filled the space. 120' radius is pretty huge, particularly if you remember it is a sphere...
Illusion magic could be used to create fog, with spells like Hallucinatory Terrain and Mirage Arcane and Major Image (cast at 6th level to become permanent) providing ideas as to how to run the effects, with you justifying how the magic came in to being (a custom spell, a wish a magic item a creature...)
There are a number of other environmental factors that can help with this, too.
As pointed out by Protonflux, fog or smoke can create an environment that would render darkvision next to useless.
You have the ability to introduce both fire and water; mix the two and you get masses of steam. In volcanic environments you could make this even more hazardous by introducing sulphur dioxide gas (known as vog - volcanic smog) etc.
Also, what about introducing rooms that have massive root systems dangling through from above.
Water-wise, if there's a lot of water coming through the ceiling, like heavy rain, that can instantly reduce visibility and make the environment much more hazardous. Also, what if the room is completely underwater, and the water is very silty?
Fire, by itself, may be from burning oil which creates thick, choking smoke. Even in a room full of torches, you'd find that the burning pitch on the torches produces a lot of smoke.
Then you could have more biological forms, such as a room full of fungal spores, with the spores being so dense that visibility is reduced to a few feet ahead.
I'm not that familiar with DnD-5E, so I wouldn't know how to go about it, but it seems to me you could achieve the same effect by reversing the conditions:
Don't go for darkness but for extreme light.
A lot of reflective surfaces (Hall of Mirrors, Crystal Cave) and a lot of extremely bright light-sources.
For bonus points have surfaces move/vibrate so the reflections waver about as well.
Good luck trying to find your way through that when you are blinded by the glare.