The Mountain Dark by Kathryn Le Veque

The Executioner Knights are back, now with one of their own falling in love with a Welsh princess who holds the key to a renewed rebellion. Will love win out over his sense of duty?
Kress de Rhydian is a man on a mission.
The second Executioner Knight is given a very important directive –William Marshal is determined to make a match between a Welsh princess and a great marcher lord. With King John having recently married his daughter to the last Prince of Gwynedd, the Marshal seeks to balance that power with a strategic marriage of his own. Kress is tasked with protecting the Welsh princess, no matter what the cost, until she can reach her destination.
The cost may be his soul.
Cadelyn of Vendotia is the daughter of two powerful royal bloodlines. Having been raised in the house of the Earls of Arundel, she is more English than Welsh, and she has no desire to be part of a political marriage. An educated woman with a penchant for naughty poetry, Cadelyn is one of the last full-blooded Welsh princesses and a very valuable commodity. But there is a complication – the princess falls in love with Kress along the way, and in spite of himself, Kress cannot deny his growing feelings for her.
But there is a rebellion afoot, one tied to her betrothed. A group calling themselves the Rebels of Rhos, led by Cadelyn’s long-lost mother, is determined to use Cadelyn to feed their cause. They have promised lands and wealth to the greedy brothers of Cadelyn’s betrothed in exchange for their support of the rebellion, one that neither William Marshal – nor Kress – have any idea about. They will be taking Cadelyn straight into a firestorm if she makes it to Wales, and the center of this storm is a place called Mynydd Twywll (Mountain Dark).
It is the heart of the rebellion.
When the terrible plans for Cadelyn finally become known, it is nearly too late to save her, but Kress and her escort give it a valiant try. When reinforcements arrive from an unexpected source, Kress risks his life to take Cadelyn to safety.
Will his sense of duty and his loyalty to England win out? Or will the love of a Welsh princess be his downfall?