Final answer:
Brabantio accused Othello of using black magic to coerce Desdemona into marriage.
Step-by-step explanation:
In Shakespeare's Othello, Sen. Brabantio, who is Desdemona's father, claims that Othello used black magic to force Desdemona to marry him. This accusation stems from a deep-seated disbelief that his daughter could genuinely love the Moor of Venice, Othello, without some sort of supernatural coercion.
Brabantio is unable to accept that Desdemona chose Othello over the more 'socially acceptable' young men of their own race and class.