Impossible Trinity is a PlayStation 3 trophy, and an Xbox 360 and PC achievement awarded to the player after completion of mission Museum Piece in Grand Theft Auto IV.

The achievement is named as such because during this mission all three main characters from the main game and the 2 downloadable episodes (Niko Bellic of Grand Theft Auto IV, Johnny Klebitz of The Lost and Damned, and Luis Lopez of The Ballad of Gay Tony, respectively) are brought together at the same place and time in an amazing coincidence.

The Impossible Trinity achievement is absent in The Lost and Damned, but returns in the Ballad of Gay Tony with a different name: "Diamonds Forever".

Trivia

• The "Diamonds Forever" achievement is an obvious reference to the James Bond film Diamonds Are Forever.
• In some cases, The "Impossible Trinity" achievement is not awarded. There is no known reason for this other than a glitch in the game or your system. Should this happen you must restart the game and play through until you get the achievement.