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A ba is the part of one's soul that represents an individual's personality and continues to live on after one's body has died. They can be summoned by Nut with Anubis' help. A ba is also known as your soul while your ren is your name and identity. The ba is one of the five aspects of the soul alongside the ka, ib, sheut, and ren.
One's ba often travels in the Duat while sleeping, often resulting in visions, particularly in the case of Magicians. A ba may also travel through time and space, such as when Sadie Kane witnessed Osiris' fateful birthday party several thousand years ago. A ba may occasionally act as a guardian. It is one of the 5 parts of the soul, like the ren, ib, ka, and sheut.
A ba often takes on the shape of some sort of bird with a human head, nicknamed chicken form by Sadie. The type of bird may vary by individual, representing one's personality.
Magicians who host gods have the ability to control their ba. Sadie, when hosting Isis could change the appearance of her ba. Her brother Carter (when hosting the spirit of Horus) could control where his went even if he was awake.
Ba travel can be prevented by using a magical headrest.
Carter's ba traveled because he didn't use the enchanted headrest that was in his room. He saw Set planning the destruction of North America with Face of Horror. When the Kanes were not at Brooklyn House, they couldn't prevent their ba from traveling. When entering a gate to the House of Life, Carter encounters a guardian ba who was once Nectanebo II, the last native pharaoh. He let him pass after saying "Go forth, good king" to Carter, much to Zia and Sadie's shock. Later Sadie encounters the ba of Iskandar as they journey into the Land of the Dead. He exchanges words with her before disappearing.
When Sadie is sleeping, she is able to accept the request to ba travel, so she might have been able to control it as her powers developed. She cannot manipulate her ba when not hosting Isis.