Black Sorceress's Revolt

Roughly 800 years ago, the tieflings and humans of Caelmarath society revolted against elven rule in the First Great Revolt, often referred to as the Black Sorceress’s Revolt after Melathea Stross, the sorceress who led the uprising. The elves brutally suppressed the revolt but the wizards carried on in secret, summoning fiendish allies and corrupting the fey roads. They carved out their lands in opposition to the elves by using dark powers. The revolt burned out within 20 years, between arcane cataclysm and the Dread Walkers and madness, but not before humans seized control of Maillon and Bourgund and built arcane strongholds at Vael Turog and at Bemmea. A strange wise woman named Baba Yaga—long known in the east— offered her services to the rebellion, calling up fortress walls overnight, unbinding elven wards, and releasing demons by the score.
The elves sought to limit the damage, but the human willingness to call on dark powers combined with the tremendous human advantage in numbers was difficult to withstand across an empire spread thinly from the Western Ocean to the Ruby Sea. The elves soon became entangled in a hopeless struggle that threatened to corrupt their hidden lands. The elves feared human contamination of the elven homeland, and for good reason. Demons, devils, and horrors could easily walk the fey roads if elven vigilance slipped, or if an elf gave in to darkness.
The records of the period say the fey roads and the Elflands became magically corrupted. The elves who became the shadow fey threw in their lot with the dark powers that the mages of Caelmarath summoned and sent against each other. The shadow elves taught even more foul magic to certain human and tiefling families, such as House Stross, House Neckart, and House Galbrion, among others. Their hideous servitor-demons were the shining children of Caelmarath, as well as the elder Dread Walkers whose strength grew as the portals between worlds were disrupted and expanded. Though the shadow elves were corrupt, most elves remained on the side of light, and they found the growing fusion of magical and mortal realms abhorrent and vile.