To get this you need to find and purchase the 3 characters listed. Important to note that you need to find the Voldemort version of Quirrell, named "Professor Quirrell (Voldemort)".
This will be one of the last Achievements you will get as it requires all 200 Gold Bricks to unlock Lord Voldemort.
Quirrell can be found in Year 1, Level 6 - Face of the Enemy. Play through until you get to the chess scene and use a Dark Magic Character on the object to the left of the board. Once destroyed you can find the token within.
Tom Riddle can be found in Year 2 Chapter 6 - The Basilisk. During the final fight with the Basilisk there is a vault in the back right corner of the room. Use Griphook on the vault and you can find the token inside.
Lord Voldemort is the most time consuming. As said earlier it requires you to have unlocked all 200 Gold Bricks. Once this is done head to Borgin and Burkes down Knockturn Alley. If you can't find it, it is about halfway down Diagon Alley, you need to move towards the screen to enter it. You will find the gold bricks inside which create a final Bonus Level. Completing this will give you the token.
Go to Madam Malkins Robes down Diagon Alley to purchase all 3 characters to unlock the achievement.