Gongmin was murdered. His 'son' might not have been his. Goryeo fell in 18 years.

King Gongmin of Goryeo was one of the last capable kings of the 474-year dynasty. He spent his reign trying to rebuild the kingdom's independence after a century of Mongol domination, purging pro-Yuan officials and reclaiming northern territory. But his personal life was as turbulent as his politics: after the death of his wife Queen Noguk, the king became unstable, and his only known son was born to a slave girl named Banya — a maid in the household of the Buddhist monk Shin Don, Gongmin's controversial advisor. The boy, named Monino, was not initially acknowledged by the king.

In 1371, Gongmin executed Shin Don. Soon after, the king accepted Monino as his son and proclaimed the boy heir apparent. The circumstances invited doubt: the boy was born in Shin Don's house, to a woman who was Shin Don's servant, and Gongmin had denied paternity before being persuaded to accept it. In October 1374, Gongmin was murdered in his sleep by palace guards, who had discovered the king's plan to kill them to conceal the paternity of another child. The eleven-year-old Monino, now King U, took the throne. Court factions questioned his parentage from his first day as king.

King U reigned for fourteen years under the shadow of illegitimacy. The court split between those who backed a king of questionable blood and those who waited for an excuse to replace him. In 1388, when U ordered General Yi Seong-gye to invade Ming China, Yi refused — his army turned back at the Yalu River, marched on the capital, and deposed U. A year later, U and his young son King Chang were poisoned on Yi's orders. A distant royal relative was placed on the throne as a puppet. In 1392, Yi Seong-gye took the crown himself, ending the Goryeo dynasty.

The 474-year dynasty did not fall to foreign invasion. It fell because a murdered king had left behind an heir whose legitimacy no one could prove — and every faction in the court put its own survival ahead of the dynastic line.