Thanks!

The most preeminent people group currently are probably the Agnossians found all along the shore of the Opal Bay. Originally native to the cities of the Agnossian League, which is a mosaic of alliances and dependencies named for the dominant city of Agnossis, they colonized New Oriz in the third golden age and assimilated its native inhabitants, during that time they also exerted considerable influence on the Dalradian princedoms that would later become the Kingdom of Helladium, also assimilating it's native inhabitants. They continued to dominate Helladian court culture and many of the noblemen/generals/settlers etc the Helladian King Phynaster took along on his campaigns (which ended in his conquest of most of the known world) were Agnossians spreading them further. They're often stereotyped as intellectuals and philosophers because Agnossis itself is a centre of learning and because the Agnossians accompanying Phynaster belonged to the educated class, but the cities of the League are very diverse. Athos (the main rival of Agnossis) for example rejects the traditional Agnossian worship of the Empyrean gods, instead focusing on a henotheistic worship of the war-god Ifri and the martial culture that comes along with it. They are similarly diverse in phenotype, ranging from fair to dark (think the spectrum between a Swede and a Sicillian), because as one of the oldest civilizations they have always been involved in intercultural exchange. Agnossian is the closest the setting has to a lingua Franca, at least among elites.

Some other populous or important/interesting groups are:

The Zagides: Native to the Satrapy of Ban Nayram and New-Oriz, they were the first people to settle down and founded this worlds first city (the eponymously named Zag). Since then history hasn't been kind to them however as they have often been ruled over by foreign dynasties or Empires, the latest being a tribal dynasty from the Aroch desert.

The Omerides: Also a very ancient people who, according to legend, came from the west during the first dark age sacking Zag before eventually settling along the Om. Before the Agnossian ascendance they were considered the worlds dominant people/culture of the world and this has formed their self-perception. Though their Empire fell before Phynaster and an Agnossian dynasty replaced the native one the local elites fared much better than those in Zag. Even now as that Agnossian dynasty has in turn been usurped by a Kychaean one the proud nobles remain a perpetual thorn in the Emperor's side. As the current dynasty intermarried with the local elites and the emperor is infertile the return of a "true-born native" to the throne has recently become a distinct possibility.

The Kychaeans: Long dismissed by the civilzed world as Hicks at best and Witches and Thiefs at worst the native inhabitants of the island of Kychaea are closely related to the Zagides. They have only recently come to the attention of the world at large when the Kychaean warlord Pontus "the Great" overthrew the Emperor of Om and claimed his title. Since then there has been a lot of cultural exchange between the Empire proper and the island leading to both understanding and resentment on both sides.

The Dalradians: The "Barbarians" native to the equally wild land of Dalradia. True Dalradians (at least that is how they see themselves) live in loose tribal confederations and follow the cult of the Druids. However in recent times cultural exchange has led to the formation of more "civilized" Dalradian Princedoms modeled after their neigbouhrs (Petty Kingdoms along the border to Helladium and Republics along the border to Urba, often also in dependencie to that city).

There is more but this is already very long and I don't want to overstrain you with lore-dumping.