The Tesdatchil was a book of black magicka. This book was written in the Elder Wilds by Lorkhan himself, before he was killed by the other Aedra.
The Tesdatchil was first used by the King of Worms, Mannimarco, to increase his knowledge of powerful magicks and to learn more about creating immortality. Mannimarco was subverted by the book's dark magic, and thus became the King of Worms. After the King of Worm's defeat at the hand of Trajan Septimus, Raminus Polus searched for the Tesdatchil, but could not find it. Believing it to be destroyed when Echo Cave collapsed, the Tesdatchil was left to be discovered by a group of Imperials. These Imperials believed they could control the power of the Tesdatchil and began to learn its secrets. Like the King of Worms, they were unable to resist the book's dark magicka, and thus became liches. After they succumbed to the book's subversion, they created the Red Wraith Cult and proceeded to wage war against the Mages Guild and the Empire of Kvatch.
When Trajan Septimus came to destroy the Tesdatchil, he too, was subverted by it and transformed. His apprentice, Hadrian Tsar, then found him in a tower in the middle of Mordas. There, after a confrontation between subverted Arch Mage and determined Arch Mage Apprentice, Trajan was able to destroy it with his powerful magicks.