How it started
Mythology
The first story of alcohol appeared in Jewang-Ungi, a Korean historical archive book.
Goguryeo kingdom
The records compiled in 1145, Samguk sagi (history of the Three Kingdoms) is one of the oldest record of Sul. The presence of Sul was given out in a story about the founding of the kingdom, Goguryeo. The story when translated to English: "Haemosu, the son of the god of heaven invited the three daughters of Habaek, a water deity, and treated them to Sul. When the three daughters were about to return home, he seduced the eldest daughter, Yuhwa, and they spent the night together. Thereafter, Yuhwa gave birth to Jumong, the future founder of Goguryeo."
Archaeological study indicates that there are records that show that these brewing skills and techniques were learned by Chinese and Silla dynasty; during the Wei dynasty of China, the governor Gasahyop of the Sadong region transferred the techniques to China and the liquor came to be known as Gokaju.
Baekje
From Wikipedia’s article on K-SOOL, shared under CC BY-SA 4.0.2