Metaphysical / Paranormal event
Cynog the Prophet foretells the return of the Three Princes who devastated the underdark and surface alike.
In Cynog the Prophet's thirteenth set of prophecies, the Three Princes are mentioned in a single line warning of their return, "When the song is silenced, the Three Princes will rise again." Unable to act on the brief and vague prophecy, tales of the demons faded in time.