The spring of 1268 opened, and still the envoys had not returned from Japan. The Koryŭ people managed to capture some Japanese from Tsushima who had come near the Korean coast. They were sent to Peking together with an envoy. The emperor was delighted, showed the captives all over the palace and reviewed the army before them. After showing them all the grandeur of the Mongol court, he sent them back to tell their king about it and to urge him to make friends with the great Yuan empire. This same year the crown prince went to the Mongol court.
Im Yun, whom the king had used as an instrument for the removal of the obnoxious Kim Chun, did not intend to go without his reward. He began to plan how he might become a king-maker himself. He desired to depose the king and put another in his place who would be quite subservient to himself. To this end he began to banish those who might oppose him in this scheme, and at last when he had cleared the way and deemed the time ripe, he surrounded himself with a powerful guard and called all the officials to a council. He told them that the king desired to kill him, but rather than die tamely he was resolved to do something desperate. He asked them if they agreed, but no man dared to open his mouth. Then putting on his armor he led the way to the palace and proclaimed Chang as king. This Chang was a distant relative of the king. He also made all the officials bow to him. The records say that this deed was accompanied by a tremendous storm of rain in which the deposed king was driven forth on foot. Im Yun and his lewd followers then proceeded to loot the palace.
The parvenu Chang, at the instance of Im Yun sent an envoy to the Mongol court saying that the king had handed over the reins of government to him. The king’s son, who had gone but lately to the Chinese court, was now on his way home. He arrived at night on the farther bank of the Yalu River and was there met by a secret messenger who had crossed in the dark to tell him that Chang had usurped the throne and that soldiers had been stationed at Eui-ju to kill him when he arrived. So the Prince turned and hastened back to the emperor and a letter was immediately dispatched demanding the reinstatement of the rightful sovereign. After two such appeals had remained unanswered the emperor threatened to send an army to enforce the demand. The officials thereupon became afraid and reluctantly put the rightful king back upon his throne. The emperor then ordered both the king and the man who had deposed him to go to China in order that the matter might be investigated. The king went but Im Yun refused and sent his son instead. The emperor ordered the king to write out the cause of the trouble but the latter feared that if he did so it would make trouble for him when he went back, for Im Yun was a powerful and unscrupulous man. He therefore told the emperor that he was troubled with a lame hand that prevented his writing. Later however, in private, he made the matter bare before the emperor and as a consequence Im Yun’s son was thrown into prison. Before returning to Koryŭ the king asked the emperor to bestow upon his son, the crown prince, the hand of one of the Mongol princesses, to give him a Mongol escort back to Koryŭ, to place a Mongol governor at P‘yŭng-yang and to return to the control of Koryŭ the northern districts of the peninsula. The emperor consented to all but the last of these requests. When the king came back to Song-do, Im Yun attempted to oppose him but was speedily put down and decapitated.