If your lucky after successfully invading an enemy you may receive their unique banner as a loot item. You can only win one banner per enemy. Each banner represents the enemy in which you won it from.
List of Banners:
As of now there is no banner for the Zombies vs. Circus invasion any of the seasonal invasions. Related Achievement
|Hall of Banners|
|Collect each enemy's banner. (Alien Banner not required)|