The Thief of Secrets

Whenever a model inflicts an unsaved wound with the Thief of Secrets, the weapon learns the weakness of that model's unit and other units of te same type (for example, if it wounds a Carnifex, it learns the weakness of all Carnifexes).  Any future attacks made using the Thief of Secrets against units whose weaknesses are known to it will successfully score a wound on a To Wound roll of 2+.  This lasts until the end of the battle.