Herag learned from some of Jim's information that the jellyfish was likely preparing something.
It initially should have suddenly appeared at Thousand Lakes and prepared to make it its territory.
So later it sent out many mermaids, intending to turn the surroundings into its domain to continue its task.
Only later did the appearance of the exorcists defeat the mermaid army, and Arios' arrival made the jellyfish realize that this place was not safe, so it chose to relocate.
No one knows the method of relocation; anyway, the jellyfish and all the mermaids disappeared into thin air.
"Did you come to me just for these things?" Herag asked after understanding.
