How to obscure the vision of players with darkvision for a short amount of time.

[NihilsticBanana]

My players are headed to a fancy ball, That's well and fine however what they don't know is that this ball is going to turn bloody in the sense that a murder will take place and every one of the strange characters my players would meet this night has a motive. I originally wanted the lights to go out so that such an event could unfold under the cover of darkness but... Well almost everyone has dark-vision. It would make for a pretty weak mystery if they see who does the killing so... What's a good alternative to obscuring the sight of my players?

[Scott Wilhelm]

Blinded, Dazzled, Dazed, Staggered, there are a lot of Conditions you could put on your players.

So, if you want to stymie characters with Darkvision at a ball, I'm thinking you should use the Glitterdust Spell. That Blinds people. It would look very reasonable. They are dancing the night away, everything is lovely, at the stroke of midnight, overhead balloons pop, and there is a shower of glitter descending on the dancers, who all coo with delight at the sparkling display until the glitter starts landing on them and people start screaming, "I can't see: I'M BLIND!" and the party realizes they are Blinded, too, and then the ninjas attack...

[SheepishEidolon]

In addition to applying conditions directly, you can change the terrain by adding mist, smoke - or a stone wall. A murderer could be smart enough to not rely on darkness only.

Alternatively, the killer could snipe or use improved invisibility - both in combination with a very high Stealth value. Which might come from a temporary effect (elixir, ally's buff, deity's boon, whatever), so the creature wouldn't have to be highly specialized or high level.

[Meirril]

A smoke-bomb would be the easiest solution. Not a lot of things can see through smoke/mist.

[Rennaivx]

Another vote here for some kind of smoke/fog effect (and that adds a nice spookiness factor). A deeper darkness spell effect should do it, too, if you want to go the darkness route.

[avr]

A mass of confused people can block vision pretty well too.

[Joey Cote]

Deeper Darkness cannot be seen through with darkvision. Have a few specially made single use items hidden about the room that are triggered at the correct time. If you are planning on having this be a mystery/investigation type situation the enchanted items can even be clues.

[TheKillerCorgi]

The Night Blindness spell specifically blocks darkvision for those affected.