Maelstrom

The Maelstrom is an odd, magical storm that lays between the Northern continents and the Southern Reaches.
It is not always visible from land or sea, but upon approach of the region in which it lays it manifests within seconds. Whatever magical or meteorlogical phenomena that is responsible for the storm is unknown. It has resisted any attempts to study or dispel it being of a power so great that even the most accomplished of magical practioiners are at a loss for how to handle it.
Approaching the storm directly is folly. The waters churn, the sky rages, and the air burns with a cacophony of thunder, lightning, waves, and cold. No vessels that have attempted any direct course through the storm have been lost and no survivors accounted for. Even skirting the limits of the storm can be a harrowing task. Storms wreak havoc upon any vessels that move within some 100km from what is believed to be the edge of the tumultuous phenomena. 
The magical nature of the environment also tends to warp creatures and objects within the storm. Sailors have reported strange sea creatures as well as flying creatures that have a semblance of similarity to natural creatures in the world, but seem somehow corrupted. The descriptions range from horrific amalgamations to creatures of myth and legend. The interesting part is that accounts of these creations often find similarities with one another on the design of such monsters, they are rarely at all the same. Either the creatures and objects continue to change as to become unrecognizable by one who had seen it before or new creations are rising up all the time. What happens with the older ones is a concern of more than one ship captain.
The Empire has decreed that all travel around the Maelstrom be restricted to approved vessels only. Naturally, pirates don't pay heed to such decrees, but are wise enough to have already known to avoid the area.