Here’s a fun dungeon room:
The room is a pool: everyone is kneedeep in water. On the other side of the pool are hooded enemies who, among their other attacks, shoot a ray that they use to dispel or suppress magic light (Light spell, sunrods, etc). The PCs must use torches or lanterns in order to see. It’s dark and shadowy and the light of the torches shines off the swirling water, making the room’s floor invisible.
The front-line enemies have an attack that disarms PCs. If you are disarmed, you have to waste time feeling around the bottom of the pool for your weapon. If the enemies disarm you of your torch or lantern, the room is potentially pitch-black.
In the middle of the room is a pit (invisible by torchlight because of the light conditions). The enemies avoid it, staying at the edges of the room. If someone falls in, they are swimming while everyone else stands, and must use move actions to climb out. Their light sources are extinguished.
Are there things in the dark water? Teeth that clamp onto swimming PCs? Maybe.
When I ran this encounter, I put away the minis. I described the blackness, the torchlight, the hiss and smell of water, and the echoes of combat in the room. No one wanted to be fighting in that room in the dark. And no one stayed in the water long enough to find out if they were alone.