Kilometers under the surface, where only the deepest caves and greediest of dwarves reach lies an empire dug out from stone by cold machines and colder rulers.
The Under Empire is ruled by Duergar, but populated by almost every sentient people in the forms of slaves. Troglodytes, drow , snivirfneblin and humans make the majority of them, but anything that can be enslaved has been by the grey skinned overlords. The duergar are pessimistic, pragmatic, and completely strangers to goodness, to the point that they misunderstand the concept to what most would call "Lawful stupidity"
The Duergar hold Laduguer as the head deity of their pantheon, with Deep Duerra, Surtur , Bhaal and Tiamat worshipped to some extent. The Halls of Laduguer is their greatest university, temple and administrative building. There, the duergar council made out of one master from the eigth crafts decide on matters of empire, be it trade, infrastructure, development, or war.
Most duergar holds are heavily fortified and built around key resources. Coal, iron, rare minerals and jewels are valued most, with cave mushroom farms, underdark lakes and strategic cave passages being secondary in use. The duergar are no stranger to digging their own roads everywhere, but whenever possible they utilize already existing caves, so that they can reallocate their shackled manpower and machines somewhere more productive.
The Under Empire is adept at creating machines of industry, having gigantic forges to make fiery machines powered by coal and magic, yet very few of the greatest ones exist. They claim to be the creators of firearms and explosives, though swords and shields are the mainstay in their armies.
Their industry is migthy, but many tasks still require slave labor, which constantly needs replenishing as they are some of the worse slave keepers. Gigantic elevators were built off to Alevaz, Armagusa ,Drazhún , Kabil Merkez, Kythin , Neueszuhause, Sabur and Varcusa in order to secure a steady supply of forced laborers. Many revolts have happened under their reign, but one of them, the "Breska Svirfelfa" almost ruined the empire. The deep gnomes had to be sealed off and gassed with massive amounts of poisons, and to this day deep gnomes will appear out of the dark and ambush any duergar not paying attention when travelling the vastly unlit cave roads.
In battle, the duergar unleash a mix heavy infantry supported by firearms and siege engines and eldritch magics, of necromancy and shadow. Their armies are small, but well trained and equipped. Toxic gasses and other poisons may be used if the force can protect itself from its effects, and every duergar hold is hell under earth for any invader , or slave attempting to break into freedom.