What accusation does Brabantio repeat against Othello to the Duke in Act 1 Scene 3?

Brabantio says to the Duke:

"To fall in love with what she fear'd to look on!
 It is a judgment maim'd and most imperfect
 That will confess perfection so could err
 Against all rules of nature, and must be driven
 To find out practises of cunning hell,
 Why this should be. I therefore vouch again
 That with some mixtures powerful o'er the blood,
 Or with some dram conjured to this effect,
 He wrought upon her."

He expresses utter shock and disbelief that his daughter, Desdemona, could possibly have fallen in love with Othello, a man she feared so much that she could not even look at him. He further avers that the mere suggestion is ludicrous and goes against the rules of nature. To him the only possible explanation for Desdemona being with Othello is that he had drugged her with a powerful mixture which overwhelmed her, or that he had forcibly given her a magical potion which affected her senses and enabled him to control her.

Brabantio had also claimed earlier that Othello had stolen Desdemona from him and that it would have been impossible for Othello to do so without using witchcraft or spells and potions which he had bought from charlatans.

The Duke displays excellent judgement by telling Brabantio that his claims cannot be believed just because he swears to them, but that they should be properly investigated. Othello then requests that Desdemona be called to give testimony. The Duke agrees and instructs that she be brought forth.

Desdemona expresses a divided loyalty to her father and Othello and says, however, that she owes her husband, Othello, greater allegiance just as much as her mother professed greater allegiance to him, Brabantio, than to her father.

Brabantio is utterly overwrought by Desdemona's statement and feels betrayed. Before departing, he tells Othello:

"Look to her, Moor, if thou hast eyes to see:
She has deceived her father, and may thee." 

Not only does Brabantio accuse Othello of using witchcraft, he also accuses him of using medicines purchased from mountebanks, (or quacks or charletons). He also acknowledges that Othello was able to put nature into disarray resulting in Desdemona being taken in by his 'sorcery,' something that she ordinarily would have been immune to.