The Fires of Vengeance by Evan Winter

Desperate to delay an impending attack by the indigenous people of Xidda, Tau and his queen craft a dangerous plan. If Tau succeeds, the queen will have the time she needs to assemble her forces and launch an all-out assault on her own capital city, where her sister is being propped up as the 'true' Queen of the Omehi.

If the city can be taken, if Tsiora can reclaim her throne and reunite her people, then the Omehi might have a chance to survive the coming onslaught.