Description
The basic gist of it is that Korea was able to subjugate/make tributaries out of the many Jurchen or Manchu tribes to its north, eventually reconquering the old lands of the Goryeo Kingdom. With their northern borders secured, they were able to enlist forces from the Jurchen nomads and invaded China during Ming's civil war with the Shun Dynasty. This led to an age of strife in China where Dynasty rose and collapsed repeatedly. Korea would conquer Beijing and Shandong, and would begin colonizing the area and assimilating it.
Eventually the Korean Empire is successful in invading Japan as well, due to their increased manpower from conquering the Chinese provinces. Korea is able to conquer Edo and raze it to the ground, shattering Japan's spirit. Korea conquers much of the coast of Japan and leaves the remaining daimyos to fight each other, while keeping a strong grasp over their politics.
On the cultural side of this timeline, Chinese Characters remain as the dominant script in Korea, due to their efforts to assimilate the huge Chinese populations in the conquered Chinese provinces. The Korean alphabet is only now growing popular, as Korea adopted the simpler Korean Script to allow for an easier increase in literacy rates. Japanese in this timeline is dominated by Kanji, with the two other alphabets being virtually extinct.