The King of Forever - Scarlet and the White Wolf, #4

The sun has returned to the Land of Night and Liall has claimed a throne he never wanted, but many still oppose the presence of his beloved Scarlet at court. While the powerful barons clamor for a royal heir to secure the succession, rumors fly of Liall's marriage to a baron's daughter, driving a painful wedge between the lovers. But the proposed marriage of Lady Ressilka isn't the only reason Scarlet has grown distant.
His magic is changing, mutating into a dangerous power he does not understand, and the only people who might be able to help him control it are the Ancients, who once tried to murder him. When the borders of Rshan are threatened and war looms, Liall agrees to a forbidden pact to save Scarlet, but the bargain may cost the Wolf his life.