16 is The Sith Lord: Have your character replace any of the Sith Lords on the main menu screen.
To qualify, you must be at max evil, as for
You must be in the Endgame, having slain the jedi Masters, including the final one on Dantoone, as for
From then on, there is a chance that the main menu will show you instead of Darth Nilus or Darth Scion. The achievement should pop as soon as you see yourself. It can be buggy, however. If it doesn't unlock immediately, try clicking load game, and then return to the main menu. Repeat until you see your character again and the achievement unlocks.