Who is Prince Heinrich XIII, and what is the Reichsbürger movement in Germany?

A group reportedly planned to destabilize Germany's government and install a self-proclaimed prince as the country's new ruler. The accusations were described as an important anti-terror operation by German Justice Minister Marco Buschmann, who tweeted that there is a suspicion that an armed attack against constitutional entities was planned.

Among the accused is an elderly man identified as "Prince Heinrich XIII," ....