You unlock the ability to do Fear Takedowns towards the end of the first Firefighter saving mission. You'll go through a tutorial when you get it as well.
To do a Fear Takedown you have to approach your enemies undetected and when prompted press
to start the takedown. You can take out multiple enemies by selecting your them with
and pressing
again as long as you have a clear line to them. (You can't continue your chain if they're around a corner or behind something) You can upgrade this ability to take out up to 5 enemies in one chain.
To recharge your Fear Takedown you must do one Silent Takedown. (
from stealth) - Thanks to Lavindathar for the information.
Take out 20 thugs by doing this and the achievement is yours.