Abridged Guide to Evil Wikia
"And so Dread Emperor Heinous thus addressed his court: ‘Are we not rulers of devils and dead, princes among usurpers? Why then should we suffer another to call himself king of our demesne?’ All agreed in this, and so war was declared upon Keter."
―Extract from the Scroll of Vainglory, thirty-ninth of the Secret Histories of Praes (destroyed by order of Dread Empress Maleficent II, only partial texts remain)[1]

Taking place at around 810 AD, a century after the end of the Sixty Years War, the three Secret Wars were the Empire’s attempt to invade Serenity - the realm of the Dead King.[2][3] It's likely the first war was launched by Dread Emperor Heinous based on the Scroll of Vainglory, though it's unknown how many Tyrants were involved overall.

On the whole, the wars were an utter disaster for Praes. The Dead King himself took to leading his army according to the Sahelians, an act without precedence as he never took to the field during the Crusades.[4]

Dread Empress Maleficent II had eventually used Demons of Absence to avert Serenity’s counter-invasion of Ater by a hellgate, roughly 25 years after the first war was declared on 835 AD.[3][5]