Revenant Piracy

They came from nowhere.
The merchant ship had been on course to Kitain, sailing from Creni with supplies of fish for trade. Ice constructs sat in the hold, keeping the product cool. Everything had been going as planned until a lance of elemental shadow magic shot through the sky, piercing through the mast. It shattered the wood, the mast falling to the side, useless.
The fog they had thought was natural eased away, controlled by another elemental mage and revealing a second ship, flying the red flag of the Oathbreakers. Before they could react, Husks were swarming the ship, having clawed their way up the side. This was why they employed fire mages, struggling to push back the shattered remains of once-people as the Oathbreakers ship grew closer.
In a moment of desperation the captain, a skilled water mage, drew waves over the surface of the ship, controlling it to only grab the Husks and throw them overboard as she forced the ship to move away from the Oathbreakers as fast as she could manage it.
When they were in open sea again, she assessed the damage. Half of her crew members were dead stone or dying, her ship was crippled, and she certainly didn't have the energy to propel it all the way back to Creni...
That was when a black shadow darkened the deck, a feral shriek echoing in the air. The captain spun, watching in horror as two wyverns landed on the deck of her ship. They bore riders, both wearing the insignia of the Oathbreakers, but needed no introduction.
The woman, pale and gaunt, wore a manic smile. Her mismatched eyes, one red one yellow seemed to glow as she dismounted from her wyvern. Lamya Muot smirked at the terrified sailors.
"You didn't really think we'd let you escape, did you?"

Summary

Revenant Piracy is a legend whispered among sailors, who all swear a friend of a friend survived an attack to tell the tale. According to the legends, the Revenant's goal isn't treasure and wealth in the traditional sense. Instead, they cripple ships and allow their Husks to board and drain the life from all those on board. They say those that aren't killed are made into Husks themselves.
More recent whisperings of Oathbreaker piracy have included wyverns in their tales. Wyvernback Riding is a common sight in the Divine Realms, and is the typical method for Knights to get around. It seems logical then, that knights who severed their bonds of oath and service to their Gods would flee on wyvernback to the surface.
Those usually dismissed as just legend by those who live landlocked, concern is spreading about the validity now that extremely similar tales are being told by airship captains.

Historical Basis

These legends seem to stem from the large number of ships that go missing in the vicinity of the The Ashen Wastes and surrounding countries. More hints come from the "husk ships" that occasionally wash ashore. These crippled vessels come aground or are towed into harbor by those seeking to salvage the ships. Once near people, however, the hull of these ships break open and Husks swarm the towns, draining all in sight before simply walking into the ocean and disappearing.
More and more reliable reports of ships flying the flag of the Revenant terrorist group, the Oathbreakers, give this legend a concerning level of realism. Maybe those old sailors were right.

Spread

This legend is extremely common in sailing communities. Superstition states that one must always gain the blessing of at least one God to ensure a safe journey, lest the Oathbreakers choose your ship to prey upon.
While seafaring communities treat this legend as fact, landlocked individuals are more skeptical.