I'm not finding a lot of information on TIL, but from what I've found and from what I remember the only criteria for the Prophecy to occur are the three signs (Fire from the Eye of Glass, the Tide of Woe, and the Bloodmoon) and that the event can only occur once per era.

Has a Bloodmoon occurred in the 4E by the time of Dragonborn? Does the Prophecy need to coincide with any other metaphysical event? I can't remember if the Nerevarine Prophecies allowed or prompted the hunt.