This is the fastest method if you want to unlock him early in your playthrough.
Once you claim the seat of an Emperor, the game will transition into its late-game phase. Achieving this rank automatically unlocks Dong Zhuo on the main menu for your next playthrough, regardless of whether he is alive or dead in your current game. Verification: How to Check Your Success total war three kingdoms unlock dong zhuo
The fastest way to unlock Dong Zhuo is to start a new campaign as or Sun Jian (both start relatively close to Dong Zhuo’s territory) and immediately march an army to fight him. You do not need to destroy his faction; you just need him to be wounded or killed on the battlefield. This is the fastest method if you want