That leave the Seneschal's Warlord Trait, Master of the Household, which allows the Warlord and all Knights within 8" to perform Reactions even though Knights normally can't unless against other Knights and super-heavies. Also gives them an additional Reaction during the opponent's shooting phase. But wait! You cry. Knights are vehicles, and they can only fire defensive weapons in a Reaction! Well, the joke's on you, Legiones Astartes player. Arm mounted weapons like that tasty thermal cannon you see on my Knight Errant always count as defensive weapons.