This will be obtained during the game but if you want it out of the way:
There are crosses on the walls of the manor in lots of places, and they make you and your powers weak. There's a cross on the wall of the dining room you can mess around with 5 times in order to quickly get this achievement.
However,
*** Spoiler - click to reveal ***In chapter 10 you will be turning a lot of crosses in order to get your tenth kill secured, as the person your killing spends the level turning the crosses back the right way.