Deathborn - Sovereigns of Bright and Shadow, #1

Corruption is a disease with no cure that ends with a rapid descent into madness and violence. And until now it only targeted mages. When an infected warden shows up challenging everything Margot thought she knew she is thrown into the chase to find the impossible cure. But to understand this new revelation she needs someone who knows possession ... She needs Nea and lucky for Margot, her warden friend Garret has been tasked with tracking the rogue necromancer down.
Garret is used to dealing with deadly mages so this should be like any other job: find the mage and deliver her to the king. But from the moment he finds Nea he is dragged into a deadly game of dark secrets and brutal machinations. Now he must make a choice: deliver Nea as promised and place a weapon in the hands of a mad man or deny his king and change the lives of mages and wardens forever.