The Dark Realm, also known as the Shadow Realm or Underworld (not to be confused with Hades' Hell). A dark land shrouded in fog and where things aren't what they seem. All shadows within our reality and other realities are connected to this place, no matter if these shadows came from the Material Plane of existence (our current reality; earth, galaxy of convergence, etc...) or other less tangible Planes (elemental planes, Arcane realm, etc...)
As such these shadows are the cause of bringing forth a mirror world. When a visitor enters this realm they may find themselves in a familiar environment and yet find it different at the same time. The mountains that you used to gaze upon from your bedroom window, may suddenly appear flipped upside down clinging to the 'ceiling' of the realm? Or the neighborhood that you live in could appear as decrepit, and decaying with its inhabitants a shadow version of themselves revealing their hidden personalities. More extreme cases would be finding landscapes of alternate realities, like per example; a reality where Helix won the war and has taken over the universe. Or a reality where civilization on Earth never existed in the first place, creating a primal Earth untouched by man.
This realm is ever-changing and the source of dark magic involving; darkness, shadows, void magic, death magic, dark energy, negative energy, etc... But one should be careful when tapping into the mystical powers of this realm, as it is easy to invite unwanted company without even knowing it and slowly get corrupted by the powers that be.
The Dark Realm is also a place where nightmares come to life, forcing visitors to be confronted by their fears or inner demons. Practitioners of the Dark Arts may be able to communicate with the creatures of this realm to receive hidden or forgotten knowledge and even grow their magic. Some creatures can even be bound into servitude, ready to be summoned in the material plane when needed. But one should be careful as these 'helpful' critters can suddenly turn against you for various reasons.
There are 2 ways to travel to the Dark Realm. The Spiritual way and the Physical way, both very dangerous.
The Spiritual way is a common journey by witch-doctors, shamans, and sometimes even necromancers. One leaves their body behind in the material plane while their 'soul' visits the dark realm. It is not recommended to do this on your own as there are threats that can trap the soul into the dark realm. And the longer you are severed from your body the greater the chances are that your soul may not be able to return to its body. If they can't find an assistant to guard their bodies in the material plane or have a tool/spell/etc... to force the soul back. Then the visitors can always try to obtain a 'spirit guide' to guide them and protect them in the dark realm. They can provide a distraction to threats while the soul returns to the body.
The Physical way refers to creating a gateway by connecting the Dark Realm with the Material Plane. Which is tricky as creatures can escape into the material plane, which could lead to devastating results. Now that both your soul and body are in the dark realm, it will attract creatures who feast upon souls or who seek a body to possess to escape the dark realm. Some even use trickery to trick a person into staying in the dark realm. One such way is by luring people with food offerings, food that's been hexed/cursed. When consuming it, it can force the soul to stay within the realm for 'x' amount of time, meaning the body is free for the taking.
Subject: The Dark Realm 
