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Iybraesil is a largely matriarchal Craftworld society, fostering many Howling Banshees shrines and female Autarchs. As followers of the blind Eldar Crone Goddess of the Underworld, Morai-Heg, the Iybraesil Eldar constantly aspire to recover the hidden secrets of the Crone Worlds, the original homeworlds of the lost Eldar empire which now reside within the Eye of Terror. Dutiful and fierce, the gifted Seers of the craftworld labour long in their hunt for the hidden secrets of the Crone Worlds. They hope to secure not only the Tears of Isha, but also ancient technologies and doomsday artefacts that will allow them to tip the balance in the war for survival. The colours of Iybraesil are blue and white. The Iybraesil Craftworld can regularly be located within the Segmentum Pacificus. The world-rune of Iybraesil, known as "Wisdom of Pain" and also the "Hand of Heg", is a reference to the Eldar myth wherein Morai-Heg, the Crone, persuaded Khaine to sever her hand so that she could drink of the wisdom of her own blood. The Eldar of that Craftworld make frequent references to this act, claiming that no knowledge is truly claimed without sacrifice.

A Crone World is one of the original homeworlds first colonised by the Eldar as they rose to power tens of thousands of standard years ago. The Crone Worlds formed the heart of the lost Eldar empire before they were consumed by the creation of the vast Warp rift called the Eye of Terror following the birth of the Chaos God Slaanesh. The worlds are now known as Crone Worlds to the Eldar after Morai-Heg, the Eldar Crone Goddess, who is also the Goddess of Fate. The entire region where the Crone Worlds were located was overtaken by the growth of the Eye of Terror, and the Eldar empire's homeworlds were consumed by Chaos and turned into nightmarish realities that now lie fully within the bounds of the Immaterium.

The Crone Worlds seem to be synonymous with the planets now designated as Daemon Worlds by the Imperium of Man. They are now domains of the Ruinous Powers and their servants along with many ancient Eldar who have been trapped within the Warp since the Fall of the Eldar. These trapped souls desperately yearn for the sweet release of death and the consumption of their soul by Slaanesh. These worlds remain vitally important to the survival of the Eldar as they are the only source for new Spirit Stones. However, journeys to the Crone Worlds constitute entering the most dangerous area of the galaxy, and many of the Craftworld Eldar who go to the Crone Worlds seeking new Spirit Stones or other valuable artefacts of their people never return. Those that do are often so badly wounded in mind and spirit that they soon seek the solace of the Infinity Circuit. Besides Spirit Stones, many other valuable artefacts of the ancient Eldar civilisation survived the Fall of the Eldar and still exist somewhere on the Crone Worlds for the taking, if any dare to claim them.