The Tome of Vethric

If your opponent's army contains any units with any of the Factions shown in the table
below, the bearer of the Tome of Vethric gains the corresponding special rule. For
example, if your opponent had a primary detachment which contained units with the Tau
Empire Faction and an Allied Detachment which contained units with the Eldar Faction,
the bearer would have both the Furious Charge and Split Fire special rules.
Enemy detachment's codex - Special rule gained by bearer
Dark Eldar - Night Vision
Eldar - Split Fire
Orks - Counter-attack
Necrons - Tank Hunter
Tau Empire - Furious Charge
Tyranids - Monster Hunter