D&D 5E Make Characters being affected from conditions without telling Players explicitly?

[Ilbranteloth]

Alternatively, you could conspire with the players against their characters.

Tell the players that some characters have been subtly charmed to behave in a certain way. Trust the players to take the idea and run with it.

In my experience, this leads to players being over-eager to screw around their characters, and on one occasion I had to rein them in a bit. "Hey, mates, its only a light charm, not a overwhelming compulsion to blindly rush into the fire."

Yeah.

I agree with [MENTION=20323]Quickleaf[/MENTION] as well, you need to have a better idea of what you're trying to accomplish. A few more thoughts, though.

The players being able to just state, "we should make saves to disbelieve" will quickly ruin something like this. One of the ways to address this is to have the save DC increase the longer they are in the region. At some point, they will need some other way to recover.

I would avoid forcing them to do something like follow the lights. They should be making those decisions. While I'm not opposed to the idea of certain situations having an effect like that (similar to frightened condition), I think that the type of scenario we're discussing will work better if you let the players run with it. That's one of the reasons you give everybody instructions, they won't know who might be affected or not.

Ideally, you have clues within the descriptions as well as other clues they can find, that will help them identify how to end the condition. Is it for as long as they are in the swamp? Using spells to protect themselves or increase their chance to save (to meet the new DCs)? Special plants or materials in the swamp that can be used to provide resistance or a cure?

Overall, I like effects that provide complications (like imposing levels of exhaustion) to make them alter their approach to adventuring. I think this works well for a location too. Especially if there is some reason they needed to go deep in the region - they could go in, suffer the start of the effects, and find a way out, realizing they need to go deeper in and find a way to deal with the disorientation, the disagreements between themselves, and also potential weakening effects. That's not hard to do if the DC is getting harder the longer they are in it (or the farther they go into the swamp or forest, etc.).

Actually, the more we discuss this, I'm going to do something similar in my campaign now. Not sure where, or exactly what, but I like what we're coming up with.