Trial by Fire — Megan Hart

Noa and Riordan declared their love for each other when she asked him to become her ahavatara, her first lover. Now Noa can summon and control the high magic, but even her newfound power isn't enough to keep Riordan by her side. When Solveig de Yourk sets her sights on having Riordan for her own, Somnus Keep is turned upside down and Noa almost loses Riordan forever.