Many are the lands of the South, beyond the southern border of Stygia, almost unknowable: a combination of willful ignorance by the mapmakers of the West and the ever-shifting borders and ill definitions of what constitutes a nation.
It may be a harsh term to our modern ears, but the phrase "The Black Kingdoms" is a common name for this region.
Although many people north of Stygia call all the Black Kingdoms by the name of Kush, Kush is the specific name of one of many kingdoms in the south.
The people in the south are the descendants of non-Thurian tribes in the south of the Thurian continent. The Cataclysm affected this part of the world less than the north, but was still devastating.
Southerners are a deeply dark-skinned people, with lithe muscular bodies and equally dark hair.
Amente, Ayele, Endale, Kagale, Kahero, Kenyatta, Mganga, Mubale, Mwando, Nastasen, Okang, Shanaka, Talharqa.
Aluna, Asminia, Chanya, Dalila, Iras, Kanika, Kenyetta, Micere, Mirembe, Nagesa, Nmambi, Nyanath, Qalhata, Sala, Sudati.
There are a number of example names in Conan the Adventurer ch.1, p.23.
https://www.fantasynamegenerators.com/mandinka-names.php
https://www.fantasynamegenerators.com/swahili-names.php
https://www.fantasynamegenerators.com/zulu-names.php
https://www.fantasynamegenerators.com/shona-names.php
https://www.fantasynamegenerators.com/ndebele-names.php
Note: I have picked these generators because they are examples of the largest language families in the real-world African continent. Just pick any name from that part of the world and let's not assign any real world civilisations to Howard's fantasy world.
Core Book ch.8, p.241.
Conan the Adventurer ch.2, p.36.
In the south of the Thurian continent were another people, a non-human race of serpentfolk.
Their unsettling ruins are found all over the Black Kingdoms.
History tells us that the founders of Stygia defeated the serpentfolk for good, but stories persist that the snakemen can still be found in the most remote parts of this region.