The Changeover - Margaret Mahy

Margaret Mahy's The Changeover is subtitled "A Supernatural Romance", and there's certainly a strong supernatural element in the book. 14 year old Laura Chant, is a New Zealand schoolgirl, living a fairly ordinary life: her parents are divorced and her mother struggles to make ends meet, working in a book shop which she isn't paid very well to manage. Laura has a 3 year old brother named Jacko. She does fairly well at school, helps out at home by taking care of Jacko when her mother has to work late, and she wonders what boys are like. However, there is something unusual about Laura: occasionally she gets premonitions about significant events that are about to happen in her life.

One day, Laura gets such a premonition and later she meets an ancient spirit who's calling itself Carmody Braque. The spirit, which is passing itself off as a Bric-a-Brac shop owner, gains power over Jacko and begins to feed off his life force, reanimating his own non-living body in the process. Whilst Braque thrives, Jacko starts to waste away. Laura sees this happening but is powerless to stop it until, in her desperation, she calls on the aid of Sorensen Carlisle, an older boy at her school, whom she suspects of being a witch.

It turns out that not only Sorry, but his mother (Miriam) and his grandmother (Winter) are also witches. They suggest that Laura become a witch herself: she is already half way there anyway, being a "sensitive". If she can survive undergoing a rather harrowing spiritual trial, Laura can "change over" to a witch, rousing the powers that are already half-awake in her, and then use them to stop Carmody Braque.

After completing the changeover, Laura makes a stamp with which to curse Braque (just as he had used a stamp, featuring his own face, to curse Jacko):

"Stamp, your name is to be Laura. I'm sharing my name with you. I'm putting my power into you and you must do my work. Don't listen to anyone but me." She thought for what seemed like a long time, though it was really only a single second, and in that time, oddly enough, the picture of the old, whistling kettle at home came into her mind. "You are to be my command laid on my enemy. You'll make a hole in him through which he'll drip away until he runs dry. As he drips out darkness, we'll smile together, me outside, you inside. We'll " (she found her voice rising higher and growing a little hysterical) " ... we'll crush him between our smiles." She looked up at the reflected witches and said nervously, "Is that enough?"
"Quite enough," Winter said, and behind the fine lace of her age, Laura saw a reflection of Sorry's wariness.
"Terrific!" exclaimed Sorry. "Chant, can I be on your side? I'd hate to be your enemy."

No wonder Winter is wary of Laura, with a curse like that !