Jangsu of Goguryeo (394–491, r. 413–491) was the 20th monarch of Goguryeo, the northernmost of the Three Kingdoms of Korea. He was born in 394 as the eldest son of Gwanggaeto. He became the crown prince in 408, and upon his father's death in 413, became the ruler at the age of 19. Jangsu reigned during the golden age of Goguryeo, when it was a powerful empire and one of the great powers in East Asia. He continued to build upon his father's territorial expansion through conquest, but was also known for his diplomatic abilities. Like his father, Gwanggaeto the Great, Jangsu also achieved a loose unification of the Three Kingdoms of Korea. In addition, Jangsu's long reign saw the perfecting of Goguryeo's political, economic and other institutional arrangements. He is also noted for building the Gwanggaeto Stele, dedicated to his father. Jangsu's posthumous name means "Long Life", based on his longstanding reign of 79 years until the age of 98, the longest reign in East Asian history. During his reign, Jangsu changed the official name of Goguryeo (Koguryŏ) to the shortened Goryeo (Koryŏ), from which the name Korea originates. Family Father: King Gwanggaeto (광개토왕, 廣開土王) Grandfather: King Gogukyang (고국양왕, 故國壤王) Unknown wife Son: Prince Joda/Juda (조다, 助多); father of King Munja. Son: Prince Seungu (승우, 升于) Unnamed daughter Early reign During his early reign, Jangsu dedicated much of his efforts toward stabilizing an empire that had experienced great and sudden growth as a direct result of his father's conquests. Jangsu built a magnificent tomb for his father, Gwanggaeto the Great, and along with it an imposing 6 meter tall tombstone engraved with his father's accomplishments (now known as the Gwanggaeto Stele). In 427, he transferred the Goguryeo capital from Gungnae Fortress (present-day Ji'an on the China-North Korea border) to Pyongyang, a more suitable region to grow into a burgeoning metropolitan capital, which led Goguryeo to achieve a high level of cultural and economic prosperity. Relations with Chinese dynasties and nomadic states When Gwanggaeto the Great ruled Goguryeo, the Chinese mainland was dominated by five non-Han Chinese peoples and divided into multiple states. During Gwanggaeto's time, Goguryeo invaded Later Yan and conquered Liaoning, but when Jangsu came to the throne, the chaos in northern China was coming to an end. The unification of northern China by Northern Wei became a crucial point for both Goguryeo and the southern dynasties of