Serpent's Skull in Eberron
With the death of King Jarot, and disagreements about the sucession to the Galifar throne, The Last War began. It began as a conflict raging between Aundair, Breland, Cyre, Karrnath, and Thrane. The conflict lasted over a century and ended with the signing of the Treaty of Thronehold in 996 YK.
Over the long years of the war the original princes and princesses of Galifar fell to treachery, assassination or old age. Their heirs took up the battle, each confident in their claim to the throne.
After a time, the Mror Holds became the first territory to declare independence and begin a series of sucessions. Over the next several decades nearly all the nations would loose territory to similar civil division or rebellion. Valenar declared itself as an independent state in 956. The Eldeen Reaches ceded from Aundair in 958. In 969 hobgoblins declared Darguun as a goblinoid state.
The war changed dramatically when, in 965, House Cannith perfected the modern warforged; a manufactured race of constructs.
The most beautiful and artistically advanced of the original Five Nations, Cyre found itself beset by a joint Brelish-Thrane invasion. Suddenly, on the second day of fighting, a cataclysm erupted in Cyre and the sky caught fire and deep fogs gathered. Battlefields were torn apart and all cities within Cyre were laid waste or simply disappeared. The conflagration consumed soldiers from all sides. All living beings were killed by choking fogs rolling across the landscape which expanded to define the current border. Devastation spread, the nearby hills became a plateau of jagged glass. To the north, a chasm opened in the earth, glowing with a cold, purple light. Magics became perverted within the Cyran borders, some even taking a life of their own. Few Cyrans were able to flee before being swallowed by the mists leaving only those currently in other lands to call themselves Cyrans.
The Treaty of Thronehold was signed in 996 YK, two years after the Day of Mourning destroyed Cyre. The treaty effectively put an end to the conflict while at the same time recognizing twelve of the fifteen new nations that had come into existence during the Last War. Warforged were also given sentient status and freed from the shackles of bondage to their makers.