We're adding a new gameplay variation to the campaign: Horde events.
Instead of simply fighting your way to the exit, some missions will challenge you to defend an energy generator against multiple waves of attackers.
The generator has limited health, so every enemy that slips through brings you closer to failure. If it's destroyed, you'll have to restart the mission.
Horde isn't a separate game mode - it is integrated into the campaign to keep the gameplay varied. Each world contains one mission featuring this objective.