The Founding

The Founding is widely considered the first age of Exandria. During this time, the gods brought forth creatures to inhabit Exandria, before splitting into two factions in the war against the Primordial Titans: the Prime Deities and the Betrayer Gods. The age ended after the defeat of the Primordials, along with the Betrayer Gods, which made the world safe for civilization to flourish.

Before the Founding
Worshipers of the Luxon believe that it predated the Founding, and that its influence was already at work in the world before the gods arrived: giving birth to the Primordials, and sacrificing most of its essence to begin a cycle of rebirth so that its children could better themselves over successive lifetimes.

The Primordials
These Elemental Titans had lain beneath the land since before the coming of the gods and now rose up to wreak havoc on the gods' creations. The great cataclysm attracted beings of both Chaos and Order. Chaotic demons arrived from an amorphous emptiness that mortals called the Abyss and gorged on the mortals' suffering. Lawful celestials came from a far-off realm.

The Protean Creators
During the Founding, gods brought forth the elves, dwarves, and humans, along with as many other races as they had the inspiration to create. Because the world was still a dangerous place, ruled by elemental chaos, many of their children were killed in the earliest days.

Saddened by the loss of so many children, the gods gifted mortals with divine magic so they might better defend themselves and tame the elements. The metallic dragons were created to aid in the defense of the Children of Creation. With the dragons' help, civilization began to flourish and mortals first offered worship (and purpose) to their creators. The gods only gained their name, form, and purpose when the mortals of this world began to worship them, though it's plausible they each had their own distinctive thoughts and feelings beforehand.

The God-Eater
Ruidus was created during the Founding as a prison for Predathos, an entity from "beyond the stars" bleeding through reality that destroyed the gods Ethedok and Vordo. Recognizing this as a threat to all life, the remaining gods and the Primordials agreed to use a portion of Exandria, possibly a continent, to trap Predathos and send it into the void. However, the prison resisted being sent away and became Exandria's second moon.

The gods seeded the folklore associating Ruidus with bad luck in order to hide its origins and to encourage mortals to avoid it, hoping that this would decrease interest in its origins and prevent Predathos from becoming a danger again. This worked for many millennia into the Post-Divergence.

The Schism
As the Primordials devastated creation, two factions arose between the gods. The Prime Deities wished to stay and conquer the Primordials, leaving the land safe for their children. The Betrayer Gods, frustrated by their losses, gave up; some gave in to madness, and some began twisting their creations and joining forces with the Abyss. The Prime Deities enlisted the help of the celestials, and the Betrayer Gods imposed order on demons and produced armies of devils. Corellon was the first to teach mortals the ways of arcane magic, so that they could defend themselves without borrowing divine power. This division was known as the Schism.

The Prime Deities, with the help of their mortal champions, ultimately destroyed the Primordials and banished the Betrayer Gods from the Material Plane. Tharizdun was imprisoned beneath Gatshadow Mountain. The Betrayer Gods had been banished and sealed so thoroughly that they could not grant their divine magic to their followers.

Melora and Pelor defeated two Primordial titans, Ka'Mort the Empress of Earth and Rau'shan the Emperor of Fire, and sealed them under Mount Ygora, which became the place of especial elemental activity tended by the Gau Drashari, growing into a holy place called Toramunda and into the cities of Cathmoíra and Avalir. The fate of one of the other defeated Primordials, an earth titan, was to be thrusted feet-first into the earth in northern Issylra, with such great force that the impact created the Zenwick Mountains.

The founding of Vasselheim
With peace finally achieved, the first true civilization was able to take root. It was called Vasselheim, the "Dawn City", and the "Cradle of Creation." This is believed to have happened tens of thousands of years before the events of Campaign 1. After the founding of Vasselheim, the races ventured forth to discover new lands. The world was named Exandria.

Civilizations other than Vasselheim
Before the age came to an end, the Kuul'tevir, noble predecessors of the lizardfolk, built cities with magic nearly on par with the elves'. An unknown civilization had a settlement in the area that became the Ivyheart Thicket. And arriving through a portal from the Far Realm, aboleths and other aberrations conquered the Underdark beneath the Bladeshimmer Coast and drove their slaves to build their capital: Salar, the Unseeable City. They slowly began expanding toward the surface, and their psychic energies gradually produced the azuremite of the Crystalfen Caverns. Not far away, Yug'Voril may have been established during this era.

The end of the era
At the end of the Founding, primeval elves felled the ferocious Molten Titan and it was sealed beneath the ground, giving birth to the Daggerbay Mountains.

Trivia

 * In conversation with Vox Machina, Highbearer Vord mentioned the words "Dawn War", speaking of the Founding, which is the name for the war between primordials and gods in the Forgotten Realms D&D lore. This might have been before Matt gave it the current name or a clever phrasing — considering Vasselheim is the only known remnant of civilization lasted since the Dawn War, its inhabitants use the relic name uncommon in the other parts of Exandria.
 * In a similar way, Vord named the Founding the "Great War of the Beginnings" and Ioun gave the Calamity the title of the last Great War.