The Boora Witch is a member of the White Martians. She had discovered a book of arcane knowledge and mastered the dark arts. This included the ability to place her consciousness in the body of another. Her own people grew fearful and exiled her to the Toonolian marshes.
Much later, the marshes was visited by Dejah Thoris and a search party, looking for precious metals. She placed her consciousness into Dejah Thoris, tried to cause a war between Helium and Zodanga and had Tardos Mors and Mors Kajak arrested so she could assume the throne. Kantos Kan discovered the truth and broke their bond when he beheaded the Witch's original body.