At the start of the first battle round, select two objective markers on the battlefield to be your foundations. When you draw a line from any part of one of your foundations to the other, if any part of a model's base (or any part of a models hull, for a model without a base) crosses that line, that model's unit is said to be on your defensive line.
While an Imperial Knights unit from your army is on your defensive line, that unit has the Against the Horde ability.
Each time one of your foundations is removed from the battlefield (e.g. by a Primary Mission rule), just after is is removed, select a new objective marker on the battlefield to be one of your foundations.
Building an army? Create, validate and share your list with New Recruit, army builder for Warhammer 40,000 10th Edition and many more games.
Try the builder