the First Age

Also called "the Age of Dragons", the First Age marks the period marked from the end of the the Divine Rebellion stretching to the First Unmaking.
Depending on whom you ask, the First Age lasted anywhere between 10,000 years and two million years. Artifacts from the First Age are rare and surviving written records from the First Age are rarer still. Most historical records known are copies of First Age documents made by historians in The Second Age.
Complicating things for historians is the numerous forgeries of First Age artifacts permeating the world because such relics are highly prized by historians and collectors alike.  All known written records were in Draconic.
What is known is dragons, were the dominant race. It is widely believed that dragons were so powerful because their ancestors fought along side the Nine during the Divine Rebellion and gained a tiny portion of Turoch's power that the Nine did not receive.
The First Age dragons were believed to be larger and stronger than modern dragons. They were definitely more numerous and organized. They formed nations, priesthoods, guilds and a variety of organized groups similar to those seen by humanoids today.
Credible sources believe there were between twenty and fifty separate dragon nations. dragons built massive towers, subterranean complexes, farms, and mines, among more exotic constructions.
Most of the dragons structures were destroyed by rampaging elementals during the First Unmaking, but a few of them intact enough to be recognizable as draconic architecture. Many elven cities in the Second Age were created in part out of salavaged ruins of draconic cities and outposts.
Surviving records and artifacts suggest that First Age dragons had a rigid social class system based on age and lineage. Details are fuzzy because much of First Age lore was passed on orally to surviving dragons and the majority of modern dragons claim to be descended from draconic royalty while their rivals are descendants of draconic peasants.
Social class often correlated to literal height with the richest and highest status dragons usually living in vast towers or mountain tops while the lowest dragons were said to have lived underground and in the sea (though modern dragons that currently live underground or in the sea will often dispute this).
One thing no one disputes is that the dragons fought each other a lot both directly and via intrigues and cold wars. dragons fought along national lines. dragons fought along religious lines. They fought over philosophies. They fought over mates. dragons fought over resources including land, food, water, magic, and treasure. The young and the old fought. The rich and the poor fought. They fought over race claiming their physical traits or breath weapon made them superior to other dragons.
Dragon-on-dragon fighting had already drastically reduced the global population and ravaged the countryside long before the First Unmaking sealed the deal ending the Age of dragons forever.