"Killer" effects, such as "Beast Killer" or "Arcana Killer", cause an enemy you are in melee combat with to sometimes be "Intimidated" (basically Paralyzed for one action) briefly, at random, while you're fighting. Most of these effects are granted by equipment such as the Shade set of armor (Empty Killer), or by inherent Job Traits. This effect can also be granted to your group for a short time if you use Circle-type job abilites.