Manipulator's War. Ruarnon Trilogy, #1

All roads lead to war. Tarlah's first nonbinary heir, Ruarnon, is determined to prove their worth to Tarlah's perfect King. But when Ruarnon's parents are abducted, they're left ruling Tarlah alone, in the shadow of impending war, with absent allies. Neighbouring King Kyura has no intention of invading. But his warmongering subjects long for the glory of expansion. When Kyura rejects their calls for war, assassins threaten his family and mutiny threatens his reign.
Across the ocean, lost Aussie and feisty history-nerd Linh seizes her only chance to get home; sailing with Ruarnon's allies. Monsters at sea threaten everyone. Linh's learnings could save Kyura's people and clear her homeward path of them. If she risks her life aiding Ruarnon. But to secure peace with Kyura's unruly subjects, Ruarnon's ultimate test as heir risks betrayal and Tarlah's bloody defeat.