Chains of Darkness, Chains of Light (The Sundered Series #4) by Michelle Sagara West
Erin of Elliath returns in this conclusion to the epic saga begun in the first three books in the Sundered series. Erin, formerly Lady Sara and now the legendary Lady of Mercy to the slaves of the Dark Empire, has just helped Renar, the rightful heir to the throne of Marantine, reclaim his kingship from an usurper. With the power of the Bright Heart waning under growing shadows of the Dark Heart, Erin and her friends must once again journey back into the Dark Empire, where High Priests battle for supremacy and the First of the Sundered, Lord Stefanos, awaits the return of his Lady Sara. In this final volume, the old ways of the Light Heart and the Dark Heart will be changed forever.