She was a member of the House Of Black an old wizarding family and one of the Scared Twenty-Eight. Cassiopeia started her education at Hogwarts School of Witchcraft and Wizardry in 1973 and was Sorted into the Slytherin House.
Sence birth she was betrothed to Argus Bulstrode,apart of the last pure-blood branch of the Bulstrode family tree and married him two years after she graduated. She soon became a death eater to please her family and her husband but regretted it greatly afterwards.
in 1997, almost three months after her husbands death in the department of mysteries, she gave birth to a baby girl whom she named: Athanasia Druella Bulstrode.
Cassiopeia was born in 1962 to Cygnus Black III and Druella Black (née Rosier), who raised her to be proud of her Pure-Blood heritage. Although as she got older she didn't really care for blood purity but hid it in fear that she would end up like her elder sister, Andromeda. After her mother cut off all contact from her to Andromeda. when she married Muggle-born wizard Ted Tonks and was disowned from the House of Black. she felt guilty because she wasn't that close to Andromeda but pretended that she never cared for her. She also was a favourite to her younger cousin Regulus.
Cassiopeia came from the Pure-Blood Black family, therefore she was given a rich lifestyle. Cassiopeia attended Hogwarts School Of Witchcraft and Wizardry and was sorted into the Slytherin House. Her parents arranged her a marriage to Argus Bulstrode, a ruthless death eater and a pure-blood, slytherin. He was from the last pure-blood branch of the Bulstrode family.
First Wizarding War (1970-1981)
During the first wizarding war Cassiopeia had started and finished Hogwarts. However she was not a death eater then nor did she fight for Voldemort's side. After Voldemort's fall on 31 October 1981, Bellatrix and a group of Voldemort's followers, including her brother-in-law Rodolphus, her other brother-in-law Rabastan, and Barty Crouch Jr, kidnapped Aurors and Order of the Phoenix members Alice and Frank Longbottom, just new parents to Neville Longbottom.
Bellatrix and her partners used the Cruciatus Curse to torture Alice and Frank for information on Voldemort's whereabouts, until the two captives were driven into insanity when they refused to divulge anything.
This crime caused a wave of fury upon the wizard community of such ferocity that the Ministry were critically pressured to find the culprits, and when the four were captured they were tried before the Council of Magical Law and sentenced to life imprisonment in Azkaban.
As a result of the torture Alice and Frank spent the rest of their lives at St Mungo's Hospital for Magical Maladies and Injuries and were unaware that they had a son together. Cassiopeia's thought's on the matter are unknown. But she might have felt guilty and saddened but acted like it was nothing like she normally did.
Between The Wars (1981-1995)
After Voldemort's fall. Cassiopeia married Argus Bulstrode. A Wealthy pure-blood and slytherin. It was him who manipulated her into joining the death eaters when Voldemort returned to power, saying that she would shame her family (Knowing that she had a constant fear of rejection). So she joined in July 1995. Cassiopeia despised Argus with passion. But remained quiet and submissive like a wife was expected to do. In a conversation with Narcissa she asks how her husband is and Cassiopeia responds with "Same as always, still sleeps like the dead and sweats like a pig" which Narcissa responds with a awkward smile and looks away. This shows Cassiopeia's inner dislike for her husband and her more 'matured' personality.
The Second Wizarding War (1995-1998)
When it was said that Voldemort returned Cassiopeia was scared but happy because she could finally become a death eater. She became one in July 1995. At the meeting in the malfoy manor, Bellatrix realised that she wasn't as 'sensitive' or 'quiet' as when they were younger.