The Invasion of the Royal Castle is a battle that took place during the Demi-Human War 43 years before the arrival of Natsuki Subaru into the world.
This battle marked a serious change for the entire war, as the three major commanders, including the undead Libre Fermi, attempted to take the Royal Castle for themselves. However, despite their efforts, they were unable to do so. Libre was completely destroyed, Valga Cromwell was defeated and was imprisoned beneath the mansion of Miklotov McMahon, and Sphinx lost her limbs and escaped, only to be secretly captured by Leip Barielle.
Despite this battle being so major, the war did not end, and the Demi-Human Alliance continued fighting under the banner of Cragrell Dawson.