The Broken Kings. Book 3 of the Merlin Codex

The hill below the fortress of Taurovinda, stronghold of Urtha, High King of the Cornovidi, is coming alive: the Otherworldly realm of the Shadows of Heroes are claiming the land as their own. But this time their actions are driven by a force that is darker and older than even the oldest among the ghosts. Who or what is raising the Dead? Merlin, a temporary resident in the fortress, must answer that question if this time of kings and lovers is to be saved.
Jason's wonderful ship, Argo, has returned, drawn back by her own guilty past, and a dreadful secret that she will reveal only to Merlin. Argo holds the key to the mystery. And Niiv, the bewitching, beautiful Northlands enchantress, is working her way even closer into Merlin's charms. This is a journey that will take Merlin back in history, and to a fabled island of legend. Love is in the air. But at a price.