Emperor's Castle (2000)

Blurb:
Chunhoo Kang lives in a world of crime, sex and intrigue. He’s the Nihon Saikono Warrior, Japan’s greatest fighter in the yakuza underworld. However, he’s also Korean. He holds the title through Shi-Nan-Joo, a powerful and ancient martial arts from Korea. Haunted by past sins, Chunhoo abandons his crime life to search for the young woman and son he abandoned decades ago. His yakuza bosses are unforgiving. They want his honor and title back. Assassins trail him all the way back to Korea and Chunhoo resolves that he’s not going to be anybody’s servant anymore. He’s going to become an Emperor, standing at the apex of his own empire. Meanwhile, his son Sukgi is a rising hotshot in the Baekhwa criminal organization in Korea. He’s got it all; money, women and power. But when the Baekhwas spark a national murder scandal, the mob boss forces Sukgi to publicly assume the guilt for the crime by threatening his mother. To save her life, Sukgi must agree to take the fall, a certain death sentence. Will father and son find each other? What will happen to Sukgi’s mother? Will Shi-Nan-Joo be powerful enough for them to survive the murderous intrigues of the crime world surrounding them? And who is the Emperor that Chunhoo seeks to become? Will he be for good or evil?

Source: www.anisearch.com/manga/3588