Pretty much anybody evil. Tom Riddle, Voldemort, Snape.
3. How do I use it?
Dark Wizards have a different kind of WIngardium Leviosa that can activate purple sparkles as well as the evil red sparkles.
4. Who is the easiest dark magician to get?
Firstly, you have to have the reducto spell. The orange blow up one. You have to get very near the end of the QUiddich World Cup Level, when you get near the caudron. If you walk south, you will find a locked chest that you must use reducto on to break the lock. YOu will get Barty Crouch Senior, who can use the dark magic.
5. Where do I find the rest?
They are mostly found using the dark magic spell. Just keep an eye out for those sparkles. ;)
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Actually the easiest for me was Tom Riddle. You can get him using Griphook (the goblin with the key) on the vault in the last chapter of Chamber of Secrets " The Basilisk". It's at the boss fight, you have to get to stage 2 where the phoenix flies in, in order to reach the vault or else Tom will just knock you away.
I forgot where I got Griphook, sorry. But he was easy to find. You need him to open any of the goblin vaults.