Sometimes even before you cast a debuf enemies have little vertical knives above their heads. Why is this? Not consistent for a certain enemy type and seems to happen at different times for different builds/characters/toons.
Probably Reduced Offensive Ability (OA). Not sure though.
Some enemies provide an aura, that causes those debuff icons over allies (some Cronley Gang heroes, for example). I believe, it’s tied with OA changes, because Veil of Shadow causes the same icon over enemy. “Broken sword” icon mens target’s damage is reduced and “broken shield” means resistances are reduced.
Yo!
So far in terms of the most common buff and debuff icons you can see in mobs (mostly when fighting skeleton type ones) or yourself:
-Grey Shield: Buff defenses and will retaliate any form of damage briefly, even DOTs.
-Red broken Shield: Defenses are lowered, even the block chance or so I think.
-Purple broken Sword: Slow down severely the movement speed.
-Red broken Sword: The damage was lowered.
-Yellow broken Sword: The attack speed is lowered. 
Not sure at all.
There are more icons but mostly they are related to curses with their unique symbols and different combination of debuffs.
Hope this helps.
Nightblade Veil of Shadows aura.
Yeah it probably is Veil of Shadows as I have had characters with night blade and usually take VoS if I am a night blade. I just never put two and two together that it was VoS.