Sarnori

The Sarnori are a people from central Essos. The territory they ruled over, the Kingdom of Sarnor, is located in the western half of what later became the Dothraki Sea. The Sarnori were one of the great civilizations of Essos, with fabulous city-states, but they were largely wiped out by the Dothraki during their great waves of conquest in the Century of Blood, which ended about 400 years before the War of the Five Kings.

The Sarnori referred to themselves as the "Tall Men" in their own language, due to their stature. They were also notably dark-skinned.

At their height, the Sarnori fielded thousands of infantry and cavalry, though their signature weapon was scythed chariots, used to cut down enemy cavalry.

In the books
No mention whatsoever was made about the Sarnori in the first five novels of the A Song of Ice and Fire series. They were first introduced in the World of Ice and Fire sourcebook (2014) which gave extensive historical background on Essos. The reason they weren't mentioned before is also straightforward: the Dothraki are repeatedly said to have conquered much of central Essos after the Doom of Valyria, but it wasn't really clear who they conquered, and logically, any prior civilizations they wiped out no longer enter into the story four centuries later during the time of the main novels. As a result of their absence from the first five novels, they did not officially enter the TV continuity at all until their history was prominently explained in the Season 6 Histories & Lore animated featurette on "The Dothraki".

The Sarnori aren't entirely extinct, as a few of their colonies survived here and there or individuals fled to other cities in Essos. They are not flourishing as they were four centuries ago but presumably Sarnori are not an uncommon sight throughout Essos even in the present day.

The Sarnori are notably non-white, very much described as African in appearance. They are described as very tall, long of limb, attractive, with brown skin, black hair and black eyes. This is more similar to how peoples from Sothoryos are described (their world's fantasy analogue of Africa), such as the Summer Islanders and Naathi. There is no hint of any historical link or migration from one continent to the other: all human nations and tribes during the Dawn Age migrated from somewhere else. Apparently the Sarnori just happened to first originate as a civilization in the west-central plains of Essos. The artistic depictions of the Sarnori in the Season 6 animated featurettes match this physical description from the World book.

While the Dothraki were mounted nomads, the Sarnori build great city-states throughout the western plains. The name "Kingdom of Sarnor" was something of a misnomer because each city-state had its own king and they frequently warred against each other (not unlike the Rhoynar). It was more of a loose affiliation or confederacy, though they would unite under a single High King against external threats. Though they used both infantry and cavalry, the Sarnori's signature weapon was scythed chariots, which they deployed by the hundreds and thousands. Sarnori apparently also considered women to be equal to men, or at least, their women fought in war just as often as men did (again, like the Rhoynar).

The Valyrians never tried to directly conquer the Sarnori.