This comes straight from the steam walkthrough wich can be found here,https://steamcommunity.com/sharedfiles/filedetails/?id=19665...
Credits to Omega Ridley
Defeat the Dread Lord's subjects in Decay.
Notes: Not a difficult achievement by any means, but it is possible to miss two of the three targets you’ll have to slay for this achievement. In Chapter 7, Decay, your task is to defeat all three Dreadwalkers, however the level is marked as completed as soon as you defeat the third one, the Dread Lord.
Both the Dread Knight, found outside of the dungeon, and the Dread Baron, found in the left wing of the dungeon, can be completely ignored. Obviously, we want the achievement, so here are the locations of all three targets on the Chapter’s map.