I mentioned in a thread a while back that I got owned by an Undin Entite in the pharos tower one time right after defeating the boss in the 2nd ascent. That entite just appeared randomly under no specific conditions. It happened right as I was about to get my ability back in the ringed section. The entite used Sleepga on everyone and put everybody to sleep. Then it started spamming Waterga until all of my guys were dead. The entite was hostile to begin with and I didn't use any magic in the area before.

I just got through the pharos again (on my current challenge), and while I was there, I saw an Undin Entite again after I defeated a boss there. This time, it was in a ringed section in the 1st ascent, the section where you can teleport to the 2nd ascent.

That entite was hostile to begin with just like that other time. Eventhough I had my gambits OFF and DID NOT use a single magic spell in that section, his life bar was still red, so it indicated that it was hostile. Right as I was about to teleport to the 2nd ascent, it used Sleepga. But right as it was about to do it, I put Nishijin Belts on everybody so they would be immune to the Sleep status effect. I evaded the sleep and then teleported on.

My theory is that whenever you're fighting a boss there, although you're put into an illusionary place, you're technically still in the same section. During the battle, you're using magic a lot. That Entite could be roaming close-by where you're fighting the boss, and it may sense that you're using that magic. That's likely why, if one is there, then it's already hostile when you encounter it.

I've also seen that same Undin Entite before in the cerobi steppe whenever it's raining hard.