The "Witches of the East" possibly alludes to the Witch of the East from The Wizard of Oz, as well as that book's many adaptations in popular culture.
For reasons yet specified, the Witches in the East came into conflict with the immensely powerful Eight Shinigami Legions. However, they were later eradicated by Asura, whom committed this act alone without his fellow warlords.
Much of the information on the Witches of the East is unknown, though whatever their ideals and/or objective warranted the attention of the Eight Shinigami Legions. In addition, while their level of power is mostly unknown but below that of Asura, Death mentioned that witches around this time period had magic that caused far more damage than in the present time.