"It's stronger than you probably fancy." He moved the book away and tutted lightly, not giving Sirius the attention that he wanted. "It's blood magic. In a sense of it but more playful. I suppose muggles call it 'blood brothers'. I know boys at the Spinner's End playground who do it but this is more powerful." If muggles could do it, why couldn't two wizards? Well, the implication was much darker but Sirius didn't have to know that.

Sirius didn't need to know a thing. "It just means we'll always be friends. Brothers. I thought it'd be nice but..."