Here is an example of such an event: http://www.youtube.com/watch?v=53kp6eduyE4 I am not sure how exactly it works but I think that only 20 to 30 enemies can be active at one time, but more will respawn if you kill them. They also include enemies with yellow health bars and shields (like Ascendant Knights). It can happen in most areas where two enemy factions are fighting each other.