If a model with the Killing Blow special rule rolls a 6 To Wound, they automatically slay their opponent – regardless of the number of wounds on the victim's profile. Armour saves and Regeneration saves cannot be taken against a Killing Blow. Ward saves may be taken as normal.
Killing Blow is only effective against Infantry, Cavalry and War Beasts. Against other troop types, a successful Killing Blow only inflicts one Wound which Ignores Armour Saves and Regeneration.
Note that if a Killing Blow attack wounds automatically, then the Killing Blow special rule does not come into play. Unless otherwise specified, Killing Blow only applies to close combat attacks.
For the purposes of combat resolution, successful Killing Blows score the same amount of Wounds as the slain model had remaining until the point of the Killing Blow was inflicted.
If a model has the Killing Blow special rule from two or more different sources (such as a combination of different special rules, spells or magic items), its effect will take place on the To Wound roll of a 5+ rather than a 6.
Heroic Killing Blow
Heroic Killing Blow functions exactly like a normal Killing Blow, except it works on any Troop Type except for Swarms. Models with Immunity (Killing Blow) treat Heroic Killing Blow as normal Killing Blow.