Son of the Morning by Linda Howard

Travel back in time to 14th-century Scotland with a scholar named Grace St. John, who discovers the missing link to a lost Celtic treasure. When she deciphers the intriguing legend of the Knights of Templar, she becomes the target of a ruthless killer. To save herself, she must enlist the help of a celebrated warrior bound by duty to uphold the Templar's secret forever. Follow Grace on her journey to confront Black Niall, a fierce man of dark fury and raw, unbridled desire, in this exceptional time-travel paranormal romance.