When you resolve an ability, you must do as much of the ability as possible, and skip any parts of the ability you are unable to resolve. For example, the Ooglan’s Parasite ability says: “Defeat an opposing basic troop in a region the Ooglan occupy. Add 1 Horde from the discard to that region.” If you had no Hordes in the discard, you could still carry out the first part of this ability, and you would just skip adding a Horde to the region since there are none to add.
