The Outer Mystery of House Bjornaer is the mystery of the Heartbeast. The Heartbeast is the word that Bjornaer magi use to describe the animal form they can change into.
Members of House Bjornaer believe that human are made up of three parts, a material body, an immaterial spirit, and an immortal soul. The spirit forms a link between the body and the soul and controls a person's animalistic passions, their instincts, and their fears. Every person has a spirit and Bjornaer magi believe the their Heartbeast is an active expression of their spirit. The secret taught to Bjornaer magi at the Ritual of Twelve Years awakens the dormant ancestral spirit and puts it on equal footing as the immortal soul, allowing both of them to affect the material body. When the soul is in control, the magus is in human form, but when the ancestral spirit takes control the mags adpots his animal form.
Each heartbeast is deeply personal to those who have awakened their ancestral spirit and the process is something that cannot be explained but must be experienced. Bjornaer magi are more in touch with their passions and emotions than are most people and they feel that they are as much their heartbeast as they are their human form.