Quote from failwiki:
Apkallu have a communal hate based on how many you have killed. As your hate grows, so does they reactment towards White Magic and physical damage reduces dramatically. The hate also effects what TP attack they use. With no hate they always use Yawn...as hate goes up they use different TP attacks.
This is what I always heard and always believed. The new Zeni NM system shed a new light on it tho. When the Zeni NPC wants Apkallus many people rush to Mount Z. There Apkallus will be killed.
Observation:
After there is an amount of Apkallus killed they start building hate. The strange thing is that even new groups arriving start hitting for 0. Also people are hitting for 0 swearing they NEVER touched an Apkallu before. I went out there with my 63 RDM alt who never touched one before and my RDM friend who finds them too cute to harm. We wwere unable to damage them from the first second on.
Test:
We changed locations. Did Ilrusi Staging point > Escape and entered the front reef. Engaging Arrapago Apkallus there made us do normal damage and they only used yawn. Besides us only 1 other group was taking pictures.
Assumption:
There is NO generic Apkallu hate at all. The mobs themself build resistance. Every Apkallu spawn area has a different hate value. After killing or damaging the Apkallus for a while all Apkallus in that spawn area will grow resistant no matter who engages them or how many apkallus you ever killed somewhere else.
Sounds good? Only way I can think of to explain my observations.