Bad Guys: Three Hundred Years Asleep, Unbeatable Upon My Return

During the reign of Emperor Taizong of the Tang, an assassin helped him eliminate his rivals before Yuan Tiangang came along. After Taizong ascended the throne, he bestowed the surname Li upon the assassin. But Li Xuantian refused to serve as an official. When Yuan Tiangang predicted that the Tang dynasty would fall, Li Xuantian volunteered to descend to the lowest level of the Ten-Thousand Corpses Cave, where he slept for three hundred years before restoring the Tang. On the night before he entered his long slumber, Taizong met with Li Xuantian in secret. No one knows what they discussed that day. Only Li Xuantian, who slept for three hundred years, knows.