here is a list of all the characters that use dark magic, to get the achievement, you need ot unlock all these characters AND their variants.
Alecto Carrow
Amycus Carrow
Bellatrix Lestrange
Death Eater
Antonin Dolohov
Fenrir Greyback
Grindelwald
Lucius Malfoy
Wormtail
Pius Thicknesse
Scabior
Snatcher
Thorfinn Rowle
Tom Riddle
Lord Voldemort
Mrs Black
Yaxley