Occasionally, in combat, a hit that I perform on a mob will pop out the word "Devastation." This seems to be distinct from a "Critical Strike," and doesn't always seem to kill the mob. Any clues?
Devastation simply means an overkill. In other words, it's when the amount of damage you do during a killing blow is much larger than the remaining HP on an enemy. This causes a devastation. I am not aware of any effect it has on the gameplay though.
Pretty sure it always kills a mob as it's one of the prerequisites of devastation. I've never seen it otherwise.