Merlin's Shadow, Merlin Spiral Series #2

Betrayal.

After destroying the sinister Druid Stone and freeing his people from its dark control, Merlin finds himself a royal advisor without a king. Along with his friend Garth and Natalenya, his betrothed, Merlin treks north with the orphaned Arthur in hopes of keeping the young ruler safe from soldiers misled by their turncoat captain. Relentlessly pursued by his old nemesis Vortigern, Merlin and his band make for the fortress of Dintaga.

But dangers multiply when Merlin realizes that Vortigern is not his only enemy. Even his own sister appears bent on Merlin’s destruction. As the threat on all their lives increases, Merlin discovers their only hope is sailing to the lands of eternal darkness and once again cleansing the world from an ancient and powerful evil.