Acropolis of Voor Darayn (5E)

From the back of the book: Centuries ago, the dark sorcerer Tur Nyaan built an extravagant palace atop a hill called Voor Darayn. For decades, he demanded tribute from nearby cities and trade routes, and summoned mighty demons or conjured terrible curses to punish those who refused him. Tur Nyaan's reign of terror ended a century ago, when death finally claimed him... or so it is believed. So why is it that summoned demons have once again appeared in the nearby cities, demanding tribute in his name? User summary: An adventure for characters of levels 5th - 9th