Alexandra Black is the only daughter of Lord Sirius Black.
Alexandra can cast wandless spells. She is very interested in necromancy, and has done several necromancy rituals. While her mother's identity is apparently known to the wizarding world to at least some degree, all we can be sure of is that her parents were unmarried.
First timeline Edit
She nearly won the Tri-Wizard Tournament.
Second timeline Edit
She joined Voldemort straight out of Hogwarts and was compared to Bellatrix Lestrange for her Dark Magic.