Blue moon rising - Simon R. Green

King John's realm has existed peacefully for generations, but now the kingdom is disintegrating. The malign rotten blackness of the Darkwood is encroaching; demons are massing, hunting in packs. King John is out of money, out of men, out of hope.
Prince Rupert, the younger son - not so much expendable as surplus to requirements - had been sent out on a Quest. When he returns, much to everyone's surprise, he has not just the princess but the dragon too . . .
A second son, a tired dragon, a unicorn without a horn and a princess with a wicked left hook: an unlikely company to face a Demon Prince - but face him they must, when the Blue Moon rises . . .