Hint is very simple. Run from the Esper. Right now, the esper was just supposed to be a menacing figure that riled up the few undead that were left after the summons (narrowing the number from like 80 to 20), and gave you guys an idea of what is to come in the story. I don't think you'll be up to facing a Dark Esper until Character level 10 or so, but I am building the story around your actions so- good luck if you continue.