Launched in the wake of Dread Empress Triumphant’s overwhelming victory and complete conquest of the continent of Calernia, the First Crusade was the only successful crusade led by a Hero aligned with the Choir of Contrition. It was waged against the Dread Empire of Praes, which at the time ruled the entire continent.
Though the details of the actual campaign are sparse, we can make several educated guesses as to what polities were involved. The Kingdom of the Dead was allied with Triumphant during the conquest and it wasn’t formally ruled by the Empire so it’s unlikely it was part of the rebelling nations. The Chain of Hunger was almost entirely slaughtered and the ratlings don't engage in diplomacy, and there’s been no records of the Everdark being invaded nor rebelling. The Golden Bloom also avoided conquest by fading into Arcadia.
The rebelling kingdoms were in that case the principalities of what later became Procer, the Kingdom of Callow, the Thalassocracy of Ashur, the Dominion of Levant and the Titanomachy. The Kingdom Under also supplied the weapons and money for the rebellion that became the crusade.
Eleanor Fairfax along with the Duchess of Daoine started the crusade. Triumphant made a habit of having children crucify their own parents as a sign of obeisance which inspired the symbol for the crusade - the red cross on a white field.
The crusade was a success and the crusading nations managed to conquer most of Praes, with the notable exception of Wolof. As her last act of spite, Triumphant collapsed the Tower and when she died, several Legions died with her.
The aftermath of Triumphant's defeat had profound consequences for the Dread Empire and the invading kingdoms both. For the Empire, it meant forty years of subjugation underneath Good, with their political institutions being destroyed and reshaped into crusader kingdoms.
It led to an establishment of a new ruling dynasty in the Kingdom of Callow - the Fairfax Dynasty - and the principalities which invaded reconvened in Salia which became the capital of the Principate of Procer.
The twenty-fourth floor of the tower features a sprawling mosaic that depicts the First Crusade.