These are sad people.
It began when they decided that the one great truth is that there isn't one.
It seemed true, at the time.
It worsened when they decided that they knew only one thing, and that one thing was that they knew nothing.
It seemed wise, at the time.
Now, they don't know if they know anything, and they don't know if they know that, or that they know that.
All they do is deny anything that is said.
This can be a very bad thing.
If you say you are friendly, you mean no harm, they will deny this, and attack you as if you were an enemy.
If you ask directions to a nearby location, they will tell you which way not to go. (This will, in fact, be the best way to get there.) Or, simply tell you that there is no such location.
If you say that you don't want to talk, they will deny this, and offer the cheerful balm of conversation. Ad nauseum.
If you say that you need to leave, they will deny this, and follow you. (They will not stop, until you state that they are following you or somesuch, at which point they will deny and leave.)
They are called the Denihilists - though they will deny this, too.
Use this instead of a monster encounter.
Put it on your encounter table.
Put them on some dank city streets, doing something totally worthless and very conspicuous.
Let me know how it goes, if you do.
For a week, I'm challenging myself to write SOMETHING gameable (monster, NPC, magic, location, item, etc.) each day. To make it interesting: it must be inspired by the first song that comes up on shuffle from my music library each morning. This is post 7, inspired by "No Paradise" by Solip (Bandcamp [FREE DOWNLOAD], Spotify).