When a friendly Cavalry model Moves, it must Charge if able. Additionally, on a turn in which a friendly Cavalry model Charges, they gain a bonus of +1 to their Strength and may re-roll any 1s during a Duel Roll that turn.
“The horn of Helm Hammerhand shall sound in the deep; one last time”
Once per game, at the start of any Move phase, you can declare you are using this special rule. Until the end of the turn, friendly Cavalry models gain the Terror special rule.