War In Sorcery's Shadow - Ruarnon Trilogy, #3

Myth says the Guardians ended the Sorcery War by draining all sorcerers of their powers, or killing them. The Guardians then vanished without a trace -and so did magic. But the myths are wrong. Magic wielders lived on in hiding. Now Sorcerer King Nartzeer is recruiting them, in a world that hates and fears their power. Terrorised by Nartzeer's pre-emptive strike, Ruarnon's Guardian-descent-claiming allies set sail to annihilate Nartzeer.
Heir Ruarnon hopes to free their family from Nartzeer's clutches first, but Ruarnon's uncle won't abandon the army he commands to slavery. Lost Aussie Linh is killing two birds with one stone, using her role as Ruarnon's envoy to potential allies to track down sorcerers to open her magic gateway home. She learns that Nartzeer is rallying a sorcery army while his neighbours mobilise against him, and sorcerers whose awesome power Nartzeer's ideology undermines, the dreaded God Kings are stirring. With sorcerers on all fronts marching towards mutually assured destruction, Linh and Ruarnon race to reach Nartzeer with the truth of magical ancestry and allegiance, hoping to unite people who are not so different before they utterly destroy each other.