The Beast of Blackmoor Bog by Kate Forsyth

The Impossible Quest, bk 3. Scholastic Australia, 2015. ISBN 9781743624081
(Age: 9-12)The quest to save Wolfhaven Castle and overthrow Lord Mortlake continues with mystery, magic and legendary creatures assisting the four children. The children find that they have no idea who to trust and that old acquaintances aren't always who they think they are.
After escaping Witchwood, and some dissention over the best way to proceed Sebastian and Elanor have come to Crowthorne Castle seeking the help of Lady Ravenna, leaving Quinn and Tom to look after the Unicorn and Griffin on the moors. It is not long before the pair come across Jack Spry, the young thief who spied for Lord Mortlake and opened the doors to allow the Bog Men into Wolfhaven castle, and find themselves in the dungeons rather than welcomed by the miserly Lady Ravenna.
They discover that Lady Ravenna has a financial arrangement with Lady Mortlake to provide the Bog Men, and that they and the other prisoners are to be fed to the bog at noon the following day as part of the Fire Festival. Elanor manages to escape the castle as Sebastian holds off the guards only to be eventually recaptured.
Meanwhile Quinn finds herself imprisoned by The Beast when she is betrayed by a girl she meets on the moors while looking for food. When Tom attempts to find her he is also locked in an oven. Fortunately Elanor is able to free them before the festivities begin and they hide in the hopes of rescuing Sebastian.
Another old acquaintance accompanies the procession to the bog, Wilda the witch, who had helped them previously and whose sight they had helped restore but who Sebastian discovers was firmly aligned with the Mortlakes.
Sebastian fears he has been abandoned by his friends but with magic, bog men, fast growing briars and man eating plants, giant leeches and a flying dragon skeleton, good triumphs and the quest continues.
Sue Keane