The War of the Hegemon.
Started when the son of the King Faurian was murdered at the Festival of Springtide by agents of the rival Kingdom of Resvayne. King Faurian took his Kingdom, Rema to war with Resvayne vowing to get revenge for his son on the Kingdom of Resvayne. The war quickly exploded into a conflict that ranged all across the penisula as other Kingdoms and City States joined into the conflict with each side garnering more and more support.
In the end it Resvayne was defeated with their King dying on Hollow's Tide Day (December 27) and his Kingdom signing a humiliating annexation treaty on December 29. The treaty became known as the Hollows Tide Treaty.