It is a vast, labyrinthine network of caves and passages, subterranean lakes and rives, the home of creatures that do not bear well the light of day. The Deep is one of the least understood - and most feared - areas of Layonara. No one, residents included, knows exactly how large the Deep is, but many believe it to be immeasurably immense. Entrances, or rumors about entrances, are known on every continent of Layonara, so the Dark Realm likely spans the globe, and is probably larger than the surface lands due to its multilayered nature.
Earthquakes, volcanic eruptions, cave-ins, mining, and burrowing by untold creatures have made a conclusive map of the Deep impossible. Not even those who have dwelt there all their lives can claim anything more than a passing knowledge of the Deep's geography. As with the corridors and caverns, entrances into this realm come and go. One might be stable for ages, then suddenly collapse, leaving no possibility of ever entering via that route again.
As for those who dwell there, the most commonly known are the Dark Elves, as they tend to raid the surface lands. Magnificent cities have been constructed in vast caves (vaults) by the Dark Elves, radiating an evil beauty in the deep darkness.
A less-known race that roams the deeps is the Deep Dwarves, or Gray Dwarves. The Deep Dwarves live in great caverns, their homes carved out from the bedrock, their smithies close to volcanic areas to take advantage of the natural heat.
Perhaps the least understood of the Deep's native cultures are the infamous Illithids, or mind-flayers. Capable of controlling other creatures' minds, they are feared by all races of the Deep. Rumor has it they devour the brain of those unlucky enough to fall before them. Their deep hatred of sunlight hardly ever leads them to the surface. Illithid cities are rumored to exist, but those who have seen them are not likely to have returned, serving as slaves or snacks to the squidlike Illithids.
Seen occasionally in ruins or caves closer to the surface are the Beholders. These large floating eyes can kill with a single gaze and thus feared by many. Little is know about their cities in the Deep, if they even have any.
The last fairly well-known race that dwells Below are the Deep Gnomes. Lovers of gems, they have managed to secure trade agreements with various surface communities. Their cities are hewn out of the bedrock near rich gem deposits, and despite their hatred of the Dark Elves (a hatred that is mutual), they have relied on stealth and cleverness, rather than aggression, to survive.
Of the populous Deep races, only Deep Gnomes refuse to practice slavery. Most of the other communities in the Deep have as many slaves as citizens.
The Deep, a place of endless wonder and terror, is not a realm to be visited by the faint of heart. No one knows exactly what is lurking in the dark.