Blue Moon Rising by Simon R. Green. It's the first of the Forest Kingdom series, and is linked to the Hawk And Fisher series. https://www.amazon.co.uk/dp/B019EKUQXW?tag=authordatabase&linkCode=ogi&th=1&psc=1

Blue Moon Rising by Simon R. Green. It's the first of the Forest Kingdom series, and is linked to the Hawk And Fisher series.

All his life, Prince Rupert thrilled in the glorious exploits of his royal ancestors. Finally embarking on his own heroic journey, Rupert realizes there’s a lot the minstrels leave out of their songs. On the hunt for a mythical dragon, besieged by demons, and navigating the cursed Darkwood with a smart-mouthed unicorn, Rupert is becoming somewhat disenchanted with legends. But even if he succeeds, peril awaits, because the king never intended for his spare heir to return from this fool’s errand.

Now, with the help of a few unusual allies, Rupert must make up the story as he goes--outmaneuvering assassins, thwarting the voracious spread of the Darkwood, and grappling with rumors of a powerful evil’s return. Rupert may not be the hero Forest Kingdom wanted, but at this rate, he’s the only one they’re going to get.