Universal Rule.
Immediately after deploying the first friendly unit of Knights of the Quest during the Deployment Phase, the owner must choose 1 unit entry from the opponent’s Army Book (this may also be a Character, but not a Character Mount). Models of all units from this unit entry in the enemy army (even if there is more than one such unit) are considered “marked”.
For attacks allocated either towards marked models, or towards models joined to units with more than half of their models marked, Questing Knights gain Lethal Strike and must reroll failed to-wound rolls.