Karl Spiesberger (29 October 1904 – 1 January 1992) was a German mystic, occultist, Germanic revivalist and Runosophist.
He is most well known for his revivalism and usage of the Sidereal Pendulum for divination and dowsing and for his anti-racialist stance and revivalist usage of the Armanen Futharkh runic system after the second world war, removing its negative connotations.
During his involvement with the Fraternitas Saturni Spiesberger was also known as Frater Eratus.
Under this name he published several articles in the Blätter für angewandte okkulte Lebenskunst.