Succession Wars

After the fall of the Imperial City to the invading demon hordes and the death of the God-Emperor, the Imperium broke up almost overnight. While the remnants of the Imperial Legion attempted to hold the demons at bay, nobles with ancient claims, power-hungry generals, demagogues, unscrupulous wizards, they all saw their opportunity to carve out their own realms from the ruins of the empire.

Of course the ambitions of these new rulers knew no bound and soon they found themselves at cross purposes. Most historians put the start of the Succession Wars at A.I. 401 after most participants have more-or-less consolidated their power. Thorin Ironfist is a notable exception as he was still fighting a civil war to take control over the dwarven kingdoms of Dorahl and Btharmaz.

The Succession Wars actually cover dozens of different engagements, where the various realms often switched sides based on what served their current purpose.

Several notable events of the Succession Wars
After almost three decades of war the continent was in ruins. Thanks to an intense diplomatic campaign by Queen Calarel of Londar the rulers of Aegeron met and signed the Treaty of Ylsalon in A.I. 429, ending the Succession Wars.
 * Archmage Carric Lunelis rallied the remnants of the Arcane Directorium and fled to Iregion where they used their arcane powers to crush the resistance of the newly independent wood-elf city states and install themselves as rulers. The wizards later started several bloody wars with their neighbors where (in the end) they summoned armies of undead and other monstrosities in the hope of gaining the upper hand.
 * Rubia and the Golden League signed a secret treaty to divide the cities of the future Tamorian Republic among themselves. Despite early successes the free cities of Tamor banded together and formed the Republic, allowing them to resist the invaders.
 * Almost every month a different city was overthrown by revolutionaries loyal to the Reaton Reds. But these revolutions were often short lived, as neighboring rulers took advantage of the situation and quickly laid siege to the already weakened cities.
 * Seeing the fight against the demons hopeless, General Titus Eldhil withdrew his forces from the Imperial City and began a long march to the west. As they marched his army raided and pillaged the lands and finally stopped at the ocean where he formed his own kingdom.
 * Racial tensions between dwarves and humans in Dwarrowdale reached a boiling point and soon the human majority started pogroms to get rid of the nonhuman population. High King Toremdir I of Cascadia sent his (overwhelmingly dwarf) forces to restore order but it ended up just further increasing the hostility of the human population. Soon Dwarrowdale announced its secession from Cascadia, which started one of the most brutal race wars of Aegeron.
 * The old human noble houses of the Imperium quickly established themselves in Minthras, Adestia and Hastogar and soon started a three-way war with each other.
 * After uniting the dwarves, Thorin Ironfist looked to the east and the dwarven armies quickly conquered the Kingdom of Minthras, which was already buckling under the combined pressure of the demon armies and their enemies to the south.
 * As the years passed the once legendary forces of the Imperial Legion were completely decimated by the demon hordes and could no longer hold them at bay. As the threat became obvious the Younger Realms put aside their differences to resist the invasion of the Abyss, but their forces proved to be inadequate. Finally, in desperation, the rulers had no choice but to accept the help offered by the Devils of the Nine Hells and sign the Brass Concordat.

Devastation of Aegeron
While the demon horde completely devastated the heartlands of the continent, the Sucession Wars also caused great damage. As the Younger Realms were all fighting for survival, they often disregarded the ages old rules of warfare and turned to abhorrent tactics (e.g. poisoning wells, unleashing plagues, summoning undead). Most scholars estimate that about a million people died during these three decades of war.