Chestnut Soldier (The Magician Trilogy) by Jenny Nimmo
Rating:This is the third book in the series, and it is also the darkest. The story is told from Gwyn's and Nia's perspectives, and the story is amazing. I say it's the darkest because an ancient evil is released early in the book, and the kids must find away to stop it.
Description:When Nia's cousin Evan comes to town strange things start happening. Something about Evan is not quite right, and Gwyn is worried that he'll cause something bad to happen. When Gwyn's wood horse is accidently released into the stream, the wrongness of Evan goes from bad to worse. Gwyn must use his magic to stop the evil and find a way to fix Evan's injured sole.