The Netherrealm is a brutal dimension that is a direct parallel to the least pleasant of afterlife worlds in various religious mythologies. It is very much hellish and is a place where evil souls go after they die to suffer eternal torment.
Out of all the realms in Mortal Kombat, this one is certainly the evilest and houses horrid monstrosities of both humanoid and inhuman nature. Appropriately, some of the darkest deeds in the franchise occur here or were plotted upon Netherrealm's burning soil before being inflicted on other realms.
7 Allegory To Underworlds
Not only is Netherrealm a direct reference to Hell from Abrahamic mythologies, but it is also connected to the "bad" afterlife options for many eastern religions as well and this can be seen in the various structures that vary in appearance from western to eastern in terms design and architecture. The souls of terrible individuals also fall down here, and this is the only time that beings are known to pass from one realm to another naturally as part of some greater plan connected to higher powers.
6 Landscape & Terrain
This realm is mostly composed of jagged and fractured masses of rocky land amidst rivers of lava and seas of blood. The skies are often depicted as yellow or orange and are clouded with smoke belched from Netherrealm's many active volcanoes. The numerous cliffs and spires make the intimidating terrain difficult to traverse, even for the demons and monsters that live here. It is also one of the two dimensions (the other being the Heavens) that is infinite in size.
5 Native Race
Unlike other dimensions that possess one or more sentient human-like species, Netherrealm's native inhabitants are actually the Oni. These monstrous organisms are very much demonic in appearance and nature as one can glean from them being named after the Japanese word for "demon". They vary quite a lot in terms of size and shape, some appearing ogre-like while others take the form of humanoid organisms with menacing and warped features.
4 Notable Oni
Although there are quite a few characters from the Mortal Kombat roster who have close ties with Netherrealm, there are only a handful that are from this plane. Moloch, the three-eyed beast who wields a portable wrecking ball in his formidable claws, is an Oni that usually appears as a mini-boss in many story modes and is sometimes a playable character who possesses a slow yet strong moveset. Drahmin, a more humanoid Oni wielding a club and sword, was once a cruel person from Outworld who died and was sent to Netherrealm to be tortured. Over countless ages, he was twisted into his current form.
The most famous of the Oni from this realm is Quan Chi, who, despite looking very human, is certainly a demonic being who simply has taken a less intimidating form for the sake of convenience. He is a necromancer capable of raising the dead and performing other unspeakable deeds of dark magic.
3 Ruler Of Netherrealm
Originally, it is mentioned that the ruler of Netherrealm was a being named Lucifer, who was also apparently its protector, much like how Raiden is the guardian of Earthrealm. However, following the war between the Thunder God and Shinnok for Earthrealm millions of years ago, the latter was cast into the underworld and eventually usurped Lucifer's title as ruler of this hellish realm. What became of Lucifer is not mentioned, however, the former Elder God of Death now sits upon Netherrealm's throne.
2 Notable Non-Natives Of Netherrealm
Of the most famous characters in the franchise, a few have become long-term residents of Netherrealm for one reason or another. Noob Saibot was cast into these depths after being slain by Scorpion, however, was not warped into an Oni. Instead, they were twisted into a wraith due to being only in a shallow layer of Netherrealm as opposed to the deeper circles which transformed Drahmin and other unfortunate souls.
Smoke, the Lin Kuei cyborg ninja, was dragged down by Noob and was twisted by the now demonic former assassin to become another wraith-like being. Scorpion, after being slain by Sub-Zero, was also sent to Netherrealm, much like Noob who he had killed, but was turned into a burning specter of vengeance.
1 Ties To Other Realms
Netherrealm is widely known by most beings in the universe as a place of eternal damnation and torment. As such, many avoid this plane as much as possible, however, Outworld is a bit of an exception. Its ruler, Shao Kahn, sometimes makes alliances with Netherrealm denizens such as Quan Chi to further his plots of conquest.
As this underworld dimension is the polar opposite of the realm known as the Heavens (the realm of the gods), it can be considered the rival and enemy plane, despite the fact that Netherrealm acts somewhat as a containment zone for undesirable souls.