Lower Realms

Some ruins or cave networks have deeper passages that are connected to the Lower Realms. The Lower Realms in Farlis spans almost the entire continent from west to east with the exception of the south west. The Lower Realms itself is a connection of large underground caverns some of which are able to sustain vast cities. It is however home to many dangerous creatures and strange vegetation making it not the most ideal place for living. The Deep Gnomes that lived however managed to maintain it's dangers until they quickly disappeared. To this day the exact reason is still unknown.
Many do not know that the Lower Realms is closely related to the elemental plane of Earth. Naturally occuring portals can be found throughout the entire Lower Realms. Many of the Deep Gnomes used these as escape routes when being invaded. It is assumed most of the current Deep Gnomes can be found on the Elemental Plane of Earth after their disappearance. It is not possible to know this for certain since all elemental portals have been closed for more than a millenium.

Geography

The Lower Realms are very varied and exits on several levels. The higher levels are the only place where expeditions are done on a regular basis. Going to the lower layers of the Underdark is very dangerous and most expeditions there have never returned. Because of this the humanoids on the surface only have an understanding of the higher layers geography. Since it is such a large area and a very varied one at that the Lower Realms is divided into many smaller areas. 

South East

The south-eastern part of the Lower Realms is the part of that lies under Terios, Varena and a part of Eprus. This area is linked to the other parts of the Lower Realms mostly by tunnels to the north. Getting to the part of the Lower Realms under Eprus is harder since it exists only out of a few entrances which are mainly protected by Dark Elf forts. The southern part of this section is the most dangerous and also home to several natural dangers.
Sulfur Flats
To the south and south east the sulfur flats form a large region blocking any further advancement. It is filled with oozes and the toxic fumes make it impossible to get through without precautions. The edges are still save to explore but most water here is boiling hot. The farthest edge of the Sulfur Flats is under Golandon which is closely linked to the hotsprings of the Anchia hill. Entering the Lower Realms from there however is still enough.

History

The main inhabitants of the Lower Realms used to be Gnomes that had descended from the ancient Gnome civilizations in Farlis. After losing their teritory in Farlis to the Precursors many of the Gnomes travelled deeper underground where they discovered the Lower Realms. Many centuries of living underground eventually turned them into the Deep Gnomes. Most of the settlements in the Lower Realms belonged to the Deep Gnomes but many have been abandoned, most likely because other creatures advanced in their territories. Other races like the kobolds and myconids lived in the Lower Realms before the gnomes but were forced to the deeper layers of the Lower Realms by the Gnomes.
When the Deep Gnomes arrived they forced many of the already present races towards the lower levels of the Lower Realms. In time the Gnomes managed to kill most of the monsterous creatures living in their part of the Lower Realms and their civlization knew prosperity. It was during this time that the Gnomes traded with the Dwarven settlements and supplied them with their technology. This allowed the Dwarves to enhance their settlements with sophisticated traps and golems.
Over the years however many other enemies had taken over parts of the Lower Realms. The first to arrive were the Dark Elves. These were Elves that fled the Precursors and decided to settle in the Lower Realms. In the beginning the relation between the races was good. As their population grew however they wanted to gain more land which caused conflict with the ruling Gnomes. Any conflicts however were quickly solved in favor of the gnomes.
The real problems started to arise when the Precursors themselves started to advance in the Underdark. After securing most of the continent the Precursors build several outposts in the higher layers of the Underdark. These outpost were often connected to their surface cities. Initially these were just mining towns and they ignored the other races living in the deeper levels. Eventually the Precursors started to attack some of the Dark Elf settlements they were forced even further into the Lower Realms which increased the tension with the Gnomes.
When the Precursors territories on the surface started to shrink some of the Dark Elves returned to the surface. Others remained behind as they considered the Lower Realms their final home. Other races started to migrate as well like the surface giants and some dragons. The Lower Realms became more and more populated.
All these migrations weakend the Gnome Empire but it would not fall until for a long time. The true downfall was started by the Nethik, a insectoid race of that was very violent. No one knows where they came from but they proved to be fatal.
The Nethik population quickly grew and the remaining myconids fled. Soon the Nethiks started expanding and their mayhem was enormous. They reproduced by infecting the dead bodies of their victims with maggots. Some of the dead bodies were even brought back in a way by turning them into Nethiks directly. The Gnome cities managed to hold their ground for a while but quickly had to give way to the waves of Nethik. Only by making full use of their golems they managed to drive back the Nethiks to the deeper levels of the Lower Realms. To this day people think that some of the golems still guard these passages to protect against any Nethik trying to move to the higher levels again. It is not known if the Gnomes were alive to see it since no Gnome civilisation is left in the Lower Realms.
Today most of the underdark is not controlled by one race but rather several enclaves. In recent years however the Dark elves have increased their terriotry and are deliberatly raiding other settlements. They have become very different from their kind on the surface with a very different religion. Even in the upper layers a hive of Nethik can be found trying to get a foothold. These are however always hunted by the other races and golems and rarely survive long. The other Nethik stay on the lower levels where most other races tend to steer away from.