Immortal Skye by K.L. Bone

Amid the Isle of Skye’s silvery waves, a vampire fell in love with a selkie.

Every vampire dreams of finding their destined mate—the one who brings the beat of life to their undead heart. Nearly two millennia ago, a pure-blood daughter of the Skye vampire bloodline found her soulmate in Rónán, the selkie knight charged with protecting her from all harm.
Rónán had saved Julia’s life when she was a child. Instead of being rewarded, he was enslaved by her father. Forced to serve the Skye family against his will, Rónán’s only comfort comes in watching over the lord’s daughter, whose innocence and sweet nature steals his heart.
Lord Skye has other plans for his only heir, including a politically advantageous pairing, and protecting her pure-blood lineage. Forced into an arranged marriage, Julia struggles to resist her father’s plans, but an ancient spell is cast upon her, rendering her helpless to the manipulations of others. As a result, Julia shatters the heart of the man she swore to love above all others.
After centuries apart, Julia is summoned home to the Isle of Skye, where she must face her past, and attempt to set right the wrongs committed against the man to whom her heart is eternally entwined.