Just read the books! This film, clearly made in an attempt to capitalise on the "Sword and Sorcery" mass market, particularly in the USA, bears no resemblance to what is, in effect, the second book of Susan Cooper's much-loved 'The Dark is Rising' sequence, which is deeply rooted in English and Celtic mythology and the landscape of the British Isles. The film shows little understanding of its source material, and deservedly bombed at the box office. Special effects are no substitute for sensitive interpretation.
If you aren't a reader, the audio version made for BBC Radio, whilst necessarily condensed, is faithful to, and respectful of, the original book.