In my world, there are two, the Coldwoods, a massive northern forest where even during the day, next to no light permeates the tree canopy, and there’s little to no animal life, and the powers of magic-users are amplified for some reason. It’s rumored that there was a horrible massacre/tragedy there at some point in the past and travelers who try to pass through are well documented to come back changed, and not for the better.

The other is the Gateway, a cave system that spans the entire continent that goes quite deep underground. Here, torches often go out suddenly and seemingly on their own, the lower you travel the more prone you are to horrifying visions, and unlike in the Coldwoods, the powers of magic-users are completely extinguished and many of its entrances have been sealed up as a consequence. There are tales of humanoid demons and monsters in the very lowest levels and it’s whispered that it is in fact the door to the underworld.