When Balthasar died on 5 April 1421, William became the sole ruler of Werle-Güstrow.
After 1426, he styled himself "Prince of the Wends, Lord of Güstrow, Waren and Werle".
William first married in 1422 to Anna of Anhalt (daughter of Albert IV, Prince of Anhalt-Köthen), who died in 1426.
[1] With her he had one child, Catherine of Werle, who married Duke Ulrich II of Mecklenburg-Stargard.
The rulers of Mecklenburg kept using the title of "Prince of the Wends" until the German monarchy was abolished in 1918.