This is a classic post and I will use these ideas
These would fit right in my Underland Setting or slip in the decent to the depths modules or underdark settings when players fall down a extra deep hole or get lost.
Im reminded of some American Indian Myths of a subterranean period of human life which probably inspired HPL's cavern dwelling races. Marvel comics has lots of warring subterranean civilizations. I had 7 lost underground cities in one of my settings and seven secret underworld civilizations. Agents of Atlas has a good secret underworld empire too.
In My current setting I also have this:I put this secretly controlled kingdom nth of Yoon Suin and it is like warring kingdom china (or the Dragon Kingdoms in Kara Tur)
Dòngxué chéngbǎo - The Cavern Citadel
A ancient eastern dynasty who built a great underground refuge and were driven into it aeons ago during dynastic struggles. The secret royal dynasty began growing addictive Black Lotus in hidden mountain valleys and used it to subvert and enslave the surface peoples, spreading a world wide drug based crime syndicate. The Royal Dynasty by occult means have become very long lived and the royal divine cult has many sects including monks, assassins, thieves, warriors and magicians. The ever living ones who make up the select of the emperor plot and scheme to become ruler. The current emprees has rulled a thousand years and has never been seen by outsiders. Those who are most dangerous are sent to the outside world to direct the empires secret empire of drugs, slavery and vice. If their service is worthy they are invited to return to the secret pleasure gardens of the hidden court if they renounce their ambition. Each empire or major land mass has such a perfect one in exile seeking to return to the subterranean paradise.
The original cavern sanctuary has expanded and has hundreds of miles of tunnels and enclaves which influence different kingdoms. Some go into the mountains, others connect to other strange kingdoms even beneath the oceans and beyond.
The crumbling Dragon Empire has broken into multiple kingdoms at the will of the Unseen Empress. The great dragon Kingdom covers the great river delta and the original empire capitol (riddled with secret buildings of the Black Lotus Cult). Two smaller empires battle their progenitor and a cluster of tiny kingdoms the thousand kingdoms against the lower edge of mountains to the south. Beyond the mountains is Yoon Suin.
Raging barbarians press against the easter edge of the three fragments of the the old empire. The three empires hate each other but stop fighting and unite vs the great khans of the wastes. The Northern empire has become steadily influenced by the barbarians culture.
The south empire has revived rituals and cults of the pre impirial bronze age dynasties and many worship spirits and monsters.
In the middle the great Dragon Empire pretends nothing has changed and assumes the lesser empires will return to the fold one day. They worship a sleepy overweight prophet who sits blissfully in a drug coma.
The Hidden Empire beneath divides the empires to make control easier and variants in imperial culture produce different luxury goods and artforms the Secret Empress desires.
The rulers of the Dragon Empires claim to be dragon descendants. Some say the true dragon born are the perfect immortals of the hidden empire beneath. According the this theory the surface empires are dupes, deluding themselves and their people.
Black Lotus is used on every continent by the rich in secret. In the western desert floodplain empires Blue lotus is more popular and acceptable. Lotus drugs are for the rich mostly but tea is enjoyed by commoners of many lands. The best tea though is quite narcotic and expensive also but rare outside the east. Elves love tea and dark elves love black lotus especially. The dark elves of the deeps help carry Black Lotus across the world, beyond the sight of shipping officials and common folk. In Shadelport a sorcerer Zorg the Great serves the Barron and controls the drug trade after a bitter battle with the hidden empire. Now the Barron's pirate fleet helps carry the drugs on the surface and into the southern continent kingdoms.
The lands of Yoon Suin produce their own drugs and tea and the Hidden empire is less interested in infiltration and manipulation of these lands. The Hidden Empress and her elite despise slug men and their elites cannot bare to even look at them.