Quote Originally Posted by Vnolan View Post
A random mob of another species needs to be killed, in addition to a number of mobs of a set species.

Source:
1. Six hours into an instance, one small group was doing crabs, another was doing worms. There were about 15 people total in the instance. I killed about two to three Zombie Wyverns and the worm spawned and after it was the crab king. Neither had spawned in the six hours prior.

2. After I was the only one left in the instance, I spawned King Igloo after killing a single Mirror Knight. King Igloo was off cooldown for a while. I let King Igloo expire. I was going to attempt to spawn Anapos with random mobs, but decided to wait a minute after Igloo in case it was blocking another NM. In that minute, the billygoat NM spawned.

3. New instance with few players. I killed 26 water sprites and switched to Matamata. Anapos spawned within three kills.

All spawns were as the mob died. The theory makes much sense where Cassie spawns are concernced: Cassie wouldn't spawn because the other mob wasn't being killed. How many groups kill Chimera or Griffins?

The community can compare data to determine further specifics.
Correlation != Causation