Forest of Fear

From the back of the book:

You're spending the summer with your uncle in Maine. He's writing a book about an old Indian legend - the story of a sacred pine tree whose branches were said to be inhabited by spirits. You decide to hunt for the Spirit Tree yourself - but when you find it you fall asleep under its branches. You wake up surrounded by six shadowy forms. They order you to make a terrifying choice: you must drink a deadly herb brew or walk across burning coals. An Indian legend is coming true - and somehow you've been trapped in it.