Mortalkind knows very little about this ancient war. All information given here is either conjecture drawn from myths and legends, tales painstakingly gleaned from the gods of knowledge, or anecdotes from the rare encounter with a dragon who survived the war.
This event was a great war that occurred about 9,000 years ago on the plane of Cahyali, fought between the "great beasts": true dragons, and the Strygarii. Little is known about this event, as most of it is in the realm of human and dwarven myth.
Dragons still exist rarely on known Cahyali, and are massive flying creatures with some sort of elemental association. Many resemble reptiles, though some appear more avian or mammalian - particularly those native to Ruefang, where they are called Long.
Though they may not truly be immortal, their lifespans are so long compared to mortals that they might as well be. There is plenty of documented evidence of dragon worship. Some people believe that dragons taught mortals magic in the time before the Stormwing Calamity.
Strygarii are not known to exist at all on known Cahyali. They are similar to dragons in that they are massive, primordial beasts associated with the world's magical elements. However, most Strygarii seem to take the form of birds of prey. Depending on their elemental associations, they are also sometimes called thunderbirds, firebirds, etc. Historians believe the Strygarii to be native to some continent beyond Esrior's Fold.
At some point long before the rise of the Dahabu Empire, dragons were the major civilization in Cahyali - though they did not build cities or leave many relics behind. Mortals were in their Stone Age, mostly living in tiny agrarian communities or living hunter-gatherer lifestyles. Most did not know magic, and both Great Beasts regarded them as little more than a passing, cute interest or a nuisance.
Dragons would eventually emigrate to the Strygarii homeland, assumedly colonizing parts of it. While things began amicably between the two Great Beasts, tensions would eventually grow. After all, one land only has so much to live on. This tension lasted for an unknown amount of time, likely multiple centuries.
The war began when a group of lightning-affiliated Strygarii, the Thunderbirds, suddenly attacked a group of dragons. No lives were lost in the incident, but the dragons retaliated in force. From here, a great war would spark that consumed most of the world in devastating battle.
Tales abound of the Strygarii killing and devouring dragons, leading some legends to write that they took pleasure in hunting them down and eating them. Certain Tarusian accounts instead state that, while the Strygarii did eat the dragons they killed in battle, it was more of an intimidation tactic: meant to send a message to the draconic forces.
Overall, the war was not much like mortal wars. There was little strategy, nor were there distinct fronts and theaters. Dragons and Strygarii clashed wherever they met, their powerful magic razing the land. Firestorms ripped across forests from Firebirds' wings, and entire segments of oceans would be frozen from ancient ice dragons' frigid breath. Both sides sustained heavy casualties, though as the war dragged on it became clear the Strygarii were winning.
At some point near the end of the war, mortals would flee the destruction. Some became lost forever on the seas, while others made it to what is now known Cahyali: the dragons' homeland. As dragons were always more sympathetic and caring towards mortals, a few legends tell of the dragons guiding mortals to Cahyali, keeping them safe through the chaos.
The dragons would eventually retreat, either fleeing through portals to other planes or back to their homeland. Those who left for other worlds would be chased and hunted down by Strygarii, and their fates remain uncertain. The remaining dragons of Cahyali would cast intensely powerful warding magic over certain points in the ocean: former battlegrounds, now with the land entirely broken or flooded.
This magic would form a bubble around the draconic homeland: targeting only the Strygarii, barring passage across. The war was over. Both Great Beasts were decimated, however; the few dragons remaining gradually retreated from mortal life, sequestering themselves in remote locations and letting mortalkind flourish without them. The status of the Strygarii, isolated beyond Esrior's Fold as they are, is unknown.
Some believe that this was the beginning of Esrior's Fold. Many more accounts, though, show that this magical ward had little physical effect. A mortal explorer could cross the waters without incident. It is instead believed that when the Fold as it is today went up, the dragons' lingering magic was used as a guide or a template to break the skies and seas.