In every day use Ka was the divine essence the pharaohs; created along with Ba at their birth.
In Ancient Egyptian mythology, the human soul is made up of seven parts: the Ren, Sekhem, the Akh, the Ba, the Ka, the Sheut, and the Sekhu.
During life, the soul, including those of animals, and of gods, was thought to inhabit a body. The Ka was the concept of life force, the difference between a living and a dead person, death occurring when the ka (in lowercase it refers to energy or something like chi) left the body. The Ka was thought to be created by the ram god Khnum on a potter's wheel, or passed on to children in their father's semen.
The Ancient Egyptians also believed that the Ka was sustained through food and drink. For this reason food and drink offerings were presented to the dead, though it was the ka within the offerings that was consumed, not the physical aspect. The ka was often represented in Egyptian hieroglyphics as a second image of the individual, leading earlier works to attempt to translate ka as double.
Like a soul, the Ba is a part of a person that lives after the body dies, and it is sometimes depicted as a human-headed bird flying out of the tomb to join with the Ka in the afterlife.
The Ancient Egyptians thought of the Ba and Ka as immortal aspects of the soul. Yet none of the three could survive if the body of the individual was destroyed.
Kazuki Takahashi's Adaptation
Takahashi's interpretations of the Ba and Ka were somewhat different. In the anime and manga, if a person's Ka was removed from their body (such as what happened to Kisara), then the person would die. The Chosen Priests powered up their Ka by using their Ba as a sort of battery. If the fuel, in this case the Ba, ran out then then they would die. This is precisely what happened to all of the Chosen Priests except Priest Seto, who survived, and Priest Mahaado, who willingly fused his Ka with his Ba, ending his life in the process.