His chance for payback came during the Mongol invasion of the country in 1283–85. Instead of defending the country, Narathihapate fled Pagan for Lower Burma in panic. The king finally became a Mongol vassal in 1287, and returned to Pagan with a small retinue. En route at Prome, on 1 July 1287, Thihathu arrested his father and forced the king to take poison. To refuse would have meant death by the sword, and with a prayer on his lips that in all his future existences "may no male-child be ever born to him again", the king swallowed the poison and died.