The Crystal Prison : Book Two of The Deptford Mice by Robin Jarvis

After vanquishing the monstrous cat Jupiter, the Deptford Mice flee their city home along with the mysterious ratwitch, Madame Akkikuyu. Together they seek refuge in the countryside with a colony of fieldmice. But as their hosts busily prepare for the great midsummer festival, something evil is stalking the cornfield in the dead of night, and soon an innocent young mouse lies murdered in the moonlight...

As the once-peaceful community is thrown into turmoil, the mice discover a terrifying enemy in their very midst, and must once again do battle with a formidable dark magic - one so powerful that even the protection of the Green Mouse might not be enough to save them.