These come with the Lamia Comandresses and Lamia No.9. Lamias almost always have Water, Ice, or Dark elementals as pets. When the Lamia's HP drops to 50% or so, they use Astral Flow...expect to take anywhere from 500-1000 damage if you're standing nearby when it happens.
If the avatars have been defeated...the Lamia will call up a new one specifically for Astral Flow, which will disappear after the attack has finished. Seems like cheating to me. -_-