In my campaign, I'm planning on having my "big bad" mess with the head of one of my PCs a bit while they sleep. The "big bad" is a very powerful witch-king type person. He is going to be trying to discourage the player character in their quest.
Specifics of the dream
The player character will start in a very gross dungeon, and then start seeing some of her friends get killed. Then, the PC will see a lot of villages and towns on fire. Then the PC will see an entire field of their friends and allies all dead, along with all the dragon Gods that the player character believes in.
They're going to see several scary things that would discourage the character. The player won't know it's a dream at the very beginning. I want this to have a negative effect on the character (like a disadvantage or something when they wake up, saying they just saw a lot of shocking things), but I don't really know what to do for the negative effect.
I was thinking the player character would have disadvantage on all Intelligence and Wisdom throws after every time the character has the dream… But then I realized that I want it to happen every night – and that's a lot of disadvantages added up. So I'm kinda stuck.
I want my player to notice the effect the dream is having on their character without making the character totally defenseless.
Is there something that can help with this?
I'm not sure whether I want the PC to wake up until they see everything that I have planned in the dream – and then I would just keep showing the same dream every night. However, I want the player to be able to sort of choose what they want to do, with some sort of disadvantage or roll they have to make (saying that it's a dream that someone else is kinda controlling).
What I was originally thinking was to have the player roll for Constitution every time they make an action beyond where I want the player to go in the dream; if the character fails, then they get a disadvantage when they wake up. However, I realize I'm running into the same problem: the amount of disadvantages she might be getting. However, I still want them to notice the effect as something real and not something to brush off. Maybe I could tone down the disadvantages; however, with even just one disadvantage every time the player has the dream, it would still add up.
How do I make this work?
Best Answer
Consider using the Dream spell
Rather than coming up with a bunch of new mechanics, you could consider using the Dream spell as a basis for what happens to the character.
If used to cause nightmares to the target, it has the effect that:
Narratively, this sounds quite a lot like what you're describing, and it's still a quite debilitating effect if the character fails her Wisdom save against the effect, since she'll take some psychic damage and won't get any benefit from her rest, which means she doesn't recover hit points, refresh spell slots or other abilities, and so on. If you like, you could still describe her as having the nightmares even if she succeeds the Wisdom save, they're just less vivid and don't affect her enough to cause her the mechanical penalties. To get more in depth, you could also give a bonus or penalty to the Wisdom save depending on what the character does in the dream; maybe successfully acting against the nightmare could grant advantage on the save, but trying and failing to take control grants disadvantage.
The spell is available to Bards, Warlocks, and Wizards, so if your big bad guy is (or is based on) one of those classes and can cast 5th level spells or has a minion who can do it for them, it's already available to them without requiring any special explanation.
As a side note, if you take this approach, I would advise caution about applying levels of exhaustion based on the character "gaining no benefit" from their rest. The core rules don't actually specify that going without a long rest causes exhaustion, only that successful long rests can reduce exhaustion levels; but Xanathar's Guide to Everything has an optional rule in the Dungeon Master's Tools chapter for going without sleep which suggests that a character who goes a day without a long rest should make Constitution saves or gain exhaustion. In this case, the character is resting, albeit poorly, and it's already harsh enough not to gain the normal rest's benefits - applying exhaustion too would be excessive, and you only have to accrue a few levels before it just kills you outright!