The Tome of Ectoclades

Once per battle, in your Command phase, you can use the Tome of Ectoclades. If you do, select one datasheet used by a unit from your opponent's army. Until the start of your next Command phase, the bearer has the following ability: 'Tome of Ectoclades (Aura): While a friendly DEATHWATCH CORE unit is within 6" of the bearer, each time a model in that unit makes an attack against a unit belonging to the selected datasheet, you can re-roll the wound roll.'