A fellow DM, and good friend of mine, is looking for help making an idea of his come to life. I've given a few ideas to him, I'd like to toss the question out to you all and see what ideas you have that I could pass on to him. His request is:

I need help figuring out dangers and "traps" or environmental effects for ghostbusting jobs in Waterdeep. The Undead monsters themselves are dangerous, but what sort of otherworldly spiritual dangers could I include to spice up the different jobs they take!

Falling net of spider web. Demonic figure that summons fire. Ground forms a mouth and swallows victim (pitfall). Poison slime oozes from walls. Giant bolder except giant's head. Walls have eyes that cast sleep or fear. Floors have teeth trying to bite (bear trap). Reaper appears to swing scythe then vanishes. Ghost hands come out of the walls/ground and grab victim. Small puddle surprisingly deep.

I get the feeling you like Halloween and macabre movies :) I like those ideas.

I hadn't even considered using vanishing phantoms, using those to cause the players to head in a different direction which is even more dangerous could be an interesting trick. I'll definitely have to give him this list, it touches on a few ideas I've given but you've got a few in there that feel like real gems.

Technically a monster, but I feel like it has some flare: A variant of Dybbuk (Mordekainen's). Give it Invisibility so it can move around without the players seeing it, and have it possess and violate the corpses of enemies they've slain (or just enemies in general). That should make a creepy, but also dangerous atmosphere.

Hmmm....

• Purely physical undead ( especially Zombies ) are decomposing corpses. There would be a possibility for purely disease transmission there.
• More ethereal undead ( Ghosts ) are likely to exist in places where the "veil between the worlds" is weak. While I don't think this would be a very common risk, there's a possibility for "leaking" through the dimensional barrier yourself, and getting lost.
• Conversely, other things can leak through and follow the Party home - especially lower Fey type creatures.
• If the interpretation in this campaign world is that undead are inherently evil. Then repeated exposure to negative undead necromantic energies could "contaminate" someone. I wouldn't make this have any direct effects on Characters - but they might start noticing that animals, creatures which react to the presence of good/evil, and some magic effects might start trigger "false positives" for evil ( a good reason to work with a Cleric who can purify/bless you after a "job" ). The Party might find themselves targeted by other slayers-of-evil, by mistake.
• It goes without saying the many undead are created by someone who might decide to take direct action against the people smashing up his/her toys.
• Large scale truly concentrated areas of undead/necromatic energy ( I'm thinking the house in Poltergeist - the original ), could be a source of all sorts of physical manifestations: storms of physical objects thrown,  sudden floods of blood ( which may, or may not, actually exist - thinking the movie The Shining here ), etc.
• Psychological attacks against the Characters - spirits latching onto the Characters, and whispering things into their ears - trying to tempt, corrupt, or drive the Characters insane. If you're familiar with playing the Shadow of another Player Character in Wraith: The Oblivion, then that sort of thing ( and a good avenue of investigation on how to do that sort of thing ). 

• Going psychological against the Players - make some of the more intelligent ( but by no means sane ) undead be sympathetic - at least on the surface. If you've read any of the Dresden Files novels, the ghost of Agatha Hagglethorn in Grave Peril is sympathetic,  right up to the point she starts smothering babies and then goes wacko and tries to kill Harry Dresden and Michael Carpenter ( and if you have not read any of the Dresden Files novels - what are you doing?! Go. Read. Now! ). If the DM wants to be really nasty make the undead ghost completely sympathetic and tragic - make the Players feel bad about dispatching her.
• Psychological against the Players - just go all in on the horror aspect. Freak your Players out! :D

Just some thoughts off the top of my head.

Sounds like fun!

Quote from SigrunNaryon >>

Technically a monster, but I feel like it has some flare: A variant of Dybbuk (Mordekainen's). Give it Invisibility so it can move around without the players seeing it, and have it possess and violate the corpses of enemies they've slain (or just enemies in general). That should make a creepy, but also dangerous atmosphere.

Oh, I'm stealing that one for my own game! I have a town that has been taken over and is now ruled by undead...this could be fun as the town's "spy master".

@Vedexent....I'll make you cringe...I've never read Dresden Files. *hides* I do know that my buddy has, so I know he'll get the references. A few of your ideas I've given him, I'm huge on the psychological horror theme, so most of the physical stuff will really help "flesh out" his game....(see what I did there)

Faerzress (from OotA / W:DH) would be an appropriate environmental hazard. I would use them the way it is detailed in Dragon Heist.

Luminous violet particles drift through the air like mist. A successful DC 12 Intelligence (Nature) check reveals that these are Underdark spores.

The purple spores are infused with faerzress, a magical radiation found in the Underdark. A creature that ends its turn in the room must succeed on a DC 13 Constitution saving throw or suffer a random form of short-term madness, determined by rolling on the Short-Term Madness table in chapter 8 of the Dungeon Master’s Guide. A creature doesn’t need to inhale the spores to be affected by them. Once the madness ends, the creature becomes immune to the spores in this room.

I will remind him of that, he's running W:DH so he can probably introduce it early, since it sounds like he's still in the early parts of the game.

By the book, it only comes up if you are running Spring (Xanathar), have to go to his lair, and enter one specific room.