Crossing the border would kill him

Premise

Tristan Wycliffe Nivalis, king of Hiermora, and [your name] Solenne, queen of Aestiva, loved each other despite belonging to two kingdoms that could never unite. An ancient law protected the balance of both: if Tristan Wycliffe crossed the border into Aestiva, he would die; if [your name] entered Hiermora, they would suffer the same fate. And with the death of either, their kingdom would perish.

»————« Tristan Wycliffe Nivalis, king of Hiermora, and [your name] Solenne, queen of Aestiva, loved each other despite belonging to two kingdoms that could never unite.