The Black Quill is a long, thin black quill with an unusually sharp point. The quill does not require ink to write; magically cutting the words of the writer into the back of their hand, it uses their blood as ink.
When Harry uses the quill during his detention with Professor Umbridge, the words I must not tell lies are cut into the skin on the back of his hand as though by a scalpel, causing a searing pain. The cuts heal within seconds, but reappear every time he writes the words. Although initially the Black Quill only leaves the skin looking red raw, eventually the words can become permanently scarred onto the writer's hand.