While there is a lot of mythos that divides the world into distinct realms of heaven and hell (such as Christianity), this isn't very accurate. The Severed Realm is as close as the actual planes get to some version of hell. This realm is where discarded souls go after they die when they cannot rid themselves of the negative emotions trying to bind them to the material plane. Since a soul that cannot release their anger, hate, envy, and other such emotions they are cut off from the other realms and trapped within this realm. Over time these negative emotions and energy will corrupt the soul and it will be transformed into a demon. These demons populate the Severed Realm and will feast upon the energy of souls which have yet to decide to give up and stay within the realm for eternity or release that which holds them to the realm and be freed.
Pull of the Negative Energy
This plane could also be considered a negative energy plane which attracts and absorbs like-energy from the cosmos. While on the material planes everything is made up of a mix of energies the contents of this plane are relatively pure in the negative energy. That is not to say negative energy in an electromagnetic sense but negative as in harmful to life - anti-life energy. This is the inverse of Aether in some way and in other ways the corruption of Aether. When the primal elemental forces which make up life become corrupted and destroy instead of give life.
All things which are tainted with such energy are drawn to this plane and its power can be used to corrupt life such as in necromancy, which draws off of this plane to animate the dead (since death is a natural part of life, reversing that through magic is a corruption of life).
Release of Negativity
To escape this realm a soul must sacrifice it's negative energy but it is impossible to do so without planing the seeds of corruption. A better way to explain it is that when inside the Severed Realm one must purge their own corruption in order to escape. A soul of the dead can do this easily as they are pure energy, separating corruption from life is easily done with no physical form. A living being, such as a person, attempting to do so will find that releasing ones own negative energy within the Severed Realm is not so easy and it comes with consequences, for the body will release the negativity that one feels and allow escape, but in doing so a new seed of corruption will be drawn in. Escaping from the Severed Realm back to the material plane is costly and can cost one dearly if the seed of corruption is allowed to take hold unchecked once the person returns to the realms of the living.
Physical Properties
The Severed Realm is experienced as a barren wasteland, hot and humid in the most uncomfortable way. There are virtually no signs of life or positive energy there. Plants do grow, in a way, but are corruptions of the material plane. Thus plants are nearly always poisonous and those that are not tend to feast of the flesh or souls of visitors. These plans that can be eaten are nearly always venomous or hold some other kind of dark and sinister way to make death an easy thing. There is running water but it is brackish and dirty filled with dangerous poisons and parasites which once they take hold can be fatal very quickly. Someone skilled in the use of the elements or with certain other uses of magical energy can purify items found to be used here. Science can also be used to determine the extend to which what is found here can be consumed without dying (given enough skill at studying poisons, parasites, and other things that kill).
Magic, Psionics, and Aether
Since this is the realm of corruption it is a side effect that the use of magical energy or psionic energy will open up a person to drawing in the negativity and corruption of this place, the effects are cumulative and a lesser fortitude of the mind would certainly be at risk of being overcome by the negative energy and corrupting force of the Severed Realm.
Death is no Release
Here, death is no sweet release and the plane actively attempts to corrupt and / or kill anyone who enters it. If one dies here, their soul could be permanently corrupted and if it is not, they will find a battle on their hands to release their negative energy before they are so corrupted that they cannot escape.