The Ildatch is a book of black magic featured in The Wishsong of Shannara. This book emerged in the Age of Faerie, long before the rise of humankind. Its story was told to Brin Ohmsford by the Druid Allanon. Allanon also told Brin that she was the only one who could destroy it.
The Ildatch claims that it was once part of The Word, a collection of secrets that gradually separated from it in time. It was first used by the rebel Druid Brona to increase his knowledge of magic. Brona was subverted by the book's dark magic and under its influence became the Warlock Lord.
After the Warlock Lord's defeat at the hands of Shea Ohmsford, Allanon looked for the Ildatch but could not find it. Although he believed it to be destroyed when the Skull Kingdom fell, the Ildatch was recovered by a band of the Warlock Lord's last remaining followers. These men believed they could control the power of the Ildatch and began to learn its secrets. Like the Warlock Lord, they were unable to resist the book's dark magic and were subverted, becoming Mord Wraiths. After they succumbed to the book's power, they took it deep into the Eastland and hid it within the Maelmord.
When Brin came to destroy the Ildatch, she too was subverted by it and was transformed. Her brother Jair Ohmsford then found her in a tower in the middle of the Maelmord. There, after a confrontation between subverted sister and determined brother which broke through the Ildatch's spell, Brin was able to destroy the tome using her wishsong.
However, one page of the book survived, as described in "Indomitable." Cogline, Kimber Boh and Jair Ohmsford set out to destroy the one page that survived Brin's magic. Using the wishsong, Jair is able to destroy the final page of the Ildatch.