Life is finally back to normal for Arthur Quinn. Three months ago, he and his friends put their lives at risk to stop the trickster god Loki from taking over the world. However, just when Arthur is starting to relax again, the dreams start once more; dreams of gods, dreams of war, dreams of wolves. It can mean only one thing. Loki is back.In the midst of a deep snowfall, Loki plots his vengeance on Arthur. In the months since their last battle, the trickster God has been assembling a deadly army of wolves and he intends to take the world once and for all. Can Arthur trust his two new classmates? Where did Ash's puppy come from? And what is hidden in the National Museum that Loki is so desperate to get? Mysteries and questions arise as, once again, it's down to Arthur Quinn and his friends to save the world. But what they don't know is that this time, Loki has help...

Arthur Quinn has defeated the World Serpent. He's come face-to-face with the Fenris Wolf. But now he faces Loki's most powerful child, Hell's Keeper. With his friends Ash, Ellie and Ex, Arthur sets out to stop this new menace. But Loki has a trick up his sleeve, a trick that changes everything.

Something wicked has awoken under the streets of Dublin ... When his dad is offered a job working on the new Metro tunnel, Arthur has to move to Dublin with him. While exploring a hidden underground river, Arthur and his new friends Will and Ash find a mysterious glowing pendant. The pendant depicts a giant snake strangling the trunk of a tree. The friends soon figure out that the pendant is a warning, a sign that something evil is waiting underneath the city. Something that's been imprisoned for a thousand years, something left by the Vikings, something that can - and will - destroy first the city, then the world. What did the Vikings bury under the city of Dublin and why did they leave it there? Who is the dark man that spies on Arthur and what is his evil plan? In the end, only Arthur and his friends can save the world from the dreaded World Serpent.