Dungeons
Dungeons are a natural phenomena that occurs when enough ambient void magic gathers in a single area. Once it has hit the natural limit of the area, it 'crystallizes' and takes a solid form as a core. Once a core is formed, the magic can no longer dissipate automatically and it sinks into the environment instead. This warps the surroundings into increasingly unnatural forms, changing the material and shape of inanimate objects. What magic that isn't grounded in this manner either mutates the plants and animals or cycles back through the core. With each cycle of ambient magic, the core will grow ever so slightly. Old and powerful dungeons have large cores at their center as a result.
The Wastelands, the largest example of this phenomena, is said to have space itself be warped several hundred kilometers in. While no one has seen the core of this dungeon, the area around it must be near unreachable if even the outskirts are this terrible.
Someone's gonna need to invent a magical ICBM to get rid of that wasteland crystal. also, reality is warped in what way? is it a nonsensical labyrinth? is everything out to kill you? are there purple bunnies that ask you impossible riddles?
A little of both A and B? The deadly aspect is mostly a side effect, the main thing is that it messes with time and space that deep in. On the light end you have days passing too fast or slow, or 'shortcuts' where space is compressed. In the center of the Wastelands, there's places where time is frozen, and you can see people going about their daily business a thousand years ago. Walking into one of those fields would be a pretty final end, and one that's impossible to get back out of.
In the end its a labyrinth that might change with the shifting currents of magic, and then you have to deal with the monsters that call them home! But think all the riches! Why wouldn't you want to go to one? Haha