This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Digimon World 3
Amaterasu Server's version of Divermon's Lake; the reddish liquid where the water would be looks very much like blood. The Divermon happily swimming in it doesn't make things better.
The Jungle Grave. The theme itself is creepy, and the fact that many players probably won't have done the Forced Level Grinding needed to simply survive one second in it means you probably got your butt kicked when you first got here.
The whole premise of getting trapped in a video game is full of Fridge Horror, especially since Digimon Online does not utilize Narnia Time. Just imagine what it would feel like to hear that your friend or familymember has essentially been trapped in a pod, their consciousness in a virtual world with no way of getting them out. One random NPC also worries about her being unable to feed her dog now. What if she (or any other pet owner playing the game) lives alone and there is nobody else around to care for her dog...?
There is also the thought of what happened to people's minds when they were turned into Oinkmon, and what happened if a random Digimon attacked them.
The whole Digimon World turning into Oinkmon is quite creepy.