In LEGO Harry Potter Years 1-4, what is the quickest way to get a Dark Wizard?

The first dark wizard I unlocked was Snape. He is in a secret area off of the potions area, taken from this guide:

You will unlock Profressor Snape once you have access to the Secret Area in the Potions room. To access this room you will need a Hufflepuff character, a book carrying character, and a character with a key. His icon is located in a chest in the Secret Area.

For what its worth, from this tip site, you can also get Lucius Malfoy easily:

One of the easiest Dark Wizard to unlock is Lucius Malfoy. Play the "Follow The Spider" level in Year 2 and unlock him when you fight the giant spider (you'll need the reducto spell).