Unforgotten (The Sisters of Kilbride 1) by Jayne Castel

A widowed clan-chief, a knife-wielding nun, and a once in a lifetime love.

Gavin MacNichol once broke Annella Fraser’s heart.
After he ended their affair to wed her elder sister, Ella fled her home and took the veil. Eighteen years later, she’s shocked to find herself face-to-face with the man she’s never forgotten, or forgiven.
Recently widowed, Gavin has lived with regret all these years. He wed for duty, not for love. But when he escorts Sister Ella of Kilbride home to visit her ailing mother, he realizes that the passion he once felt for Ella still burns—hotter than ever.
Yet Ella is forbidden. She’s sworn her life to the Cluniac order, and is determined to resist the emotional response Gavin’s nearness provokes in her.
It’s only when Ella falls foul of the tyrant who controls the fate of her beloved abbey that Gavin and Ella’s lives become entwined once more.
But, this time, their fate is one that neither can run from.