In this video guide I will show you how to defeat the Dread Lord's subjects in Decay. Now the achievement may sound a little misleading. All you need to to is, find the Dreadwalker mini bosses. If you look at your side missions, one is called Dread Knight, and the other is called Dread Baron. So find them and kill them for the achievement.
The first one is located in the top of the map in the open area north of where the snakes are in the waterfall. This is also the first floor. They do take quite a bit of damage, so make sure to use your ultimate often when it builds up.
The second one will require a new gear piece. Go into the dungeon and make your way to the right path when at the four way intersection. There you will find the gear piece for War needed to accesses the left side of the map. Once you smash the crystal, make your way around until you get a similar room like before. There will be the other Dreadwalker, kill him and get the achievement.