Please help: I want to make a horrific dungeon in D&D (4.0)

[Derjuin]

Truth be told, this is my first time DMing a campaign for two of my friends. So far they've been in an old crypt and an abandoned fortress. I'd like their next adventure to be in a dark and dreary swamp, more akin to a horror movie than an action movie. Luckily, neither player visits this website, so I can be free to ask here:

How can I make a dungeon more horrific? Getting split up might be hard to do...but slimy ooze monsters, noises in the dark/fog, and ambushes are on the list.

I hope this is enough information to go by to help; if not, I can offer more if it's needed.

[Flickerdart]

Scatter one or two Tombs of Horror about the place, that'll do the trick.

[Mando Knight]

Things that slow, immobilize, daze, and/or blind are somewhat scary, and guess what lives in swamp caves?

That's right, the DM's pesky Lurker friend, the Black Dragon. Stuns an area once per encounter, deals ongoing damage, and hides in murky darkness that not even a Drow can see through... but it can.

[HMS Invincible]

Make a cavern that is only accessible by swimming 5+ minutes through a underwater tunnel. At the end, there is a raised platform jutting out of the water. Place 4 wizards and 1 elite guard. Have them lay suppressive fire such as wall of fire and choking/stinking cloud. Anybody who gets close, push them off the platform with push spells or into the walls of fire.