Marius Black was the third child of Cygnus Black II and Violetta Bulstrode, and brother of Pollux Black, Cassiopeia Black, and Dorea Black. Marius was disowned and removed from the Black family tree tapestry for being born a Squib, a wizard-born person with no magical abilities. The Blacks were likely ashamed of his status, as Squibs are considered second-class citizens in the wizarding world and disregarded. Marius's date of birth is unknown, but given when his siblings were born, Marius was likely born between 1915 and 1920.