The upcoming adventure Princes of the Apocalypse has it's major villains in the form of the Elemental Prophets who are each creating a Temple to their Element. Also
Spoilers read ahead at your own risk.
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Anyway Chris Perkins has shared some details about the four prophets. Starting with their names. Aerisi Kalinoth the Air Prophet, Marlos Urnrayle the Earth Prophet, Vanifer the Fire Prophet, and Gar Shatterkeel the water prophet.
On details of the 4 Prophets, Gar and Aerisi are the one's with the most info about them (Which is still not much). Gar Shatterkeel despite his appearance is a human (But an abnormal one.) Aerisi the adventure's cover-girl was originally thought to be an Avariel, aka a Winged Elf. However it was recently revealed that she is actually a normal Moon Elf pretending to be an Avariel, and that her wings are illusory.
In response to the reveal of Aerisi's fake wings someone wondered about tagline for the story line "Unearth the Deception" while questioning if any of the Prophets were what they seemed to be. Chris Perkins responded with "Each used to be someone else." creating more mysteries about them.
I will update this thread with more info on the prophets as it becomes known. For now speculation is the most we can do for the four.