I Transmigrated to the Tang Dynasty and Crushed Li Jiancheng to Death from the Start

Cui Chen is involved in a car accident and unexpectedly transmigrates to the ninth year of Wude in the Tang dynasty. Worse still, it is the fourth day of the sixth month. And, by an even stranger twist of fate, he transmigrates to a spot above Xuanwu Gate. Li Jiancheng is doomed. (This is a novel, not history. Please don't nitpick if anything doesn't make sense.)