Hundreds of years ago, the Islands were home to a powerful but reclusive kingdom. They knew magic well and sailed even better. Whatever they wanted, they had, charging high taxes on anyone that crossed through their waters, but also ensuring their safe travels.
The nobles lived well indeed, pressing even higher taxes from their simple fishermen, prohibiting them from owning proper ships, keeping them dependent on them to relocate to new islands whenever the time came for an island to vanish again. There are still stories floating around of the families that were lost when they refused to take them on, for some kind of perceived slight, attack or whatever they fancied to be the excuse this time.
All of this changed when a badly damaged, but still working military ship from a long forgotten war washed ashore on a smaller island. The people there decided to keep it to themselves, something that would bring them the death penalty if they were found out. It was still a risk they were willing to take, many of them believing they were meant to be left behind anyway.
With this, a tradition of the Isles was started. If you find a ship, it is yours to keep, in whichever condition it may be. Now answer me this? What happens when desperate people get ahold of a weapon such as this? They use it and ensure that they have fighting chance left. This started a chain of attacks and raids on the different nobles on the Isles.
While they may have been fighting each other in staged battles and small ritualized skirmishes, they had not fought a proper battle in a long time, especially against an unexpected and desperate adversary. The smaller lords and ladies fell fast and the rebel fleet grew. It was not the first time this had happened, but it is said that the new fleet had a secret weapon at their side, what exactly it was is lost to time. What we know is that they always approached unsee, striking from mists that stayed unnaturally long.
The rulers were not very concerned, believing their personal fleet would be enough to quell the rebellion when they became a true problem. For a decade this raged on, enough time for the rebels to send someone that rose within the enemy ranks, a strategist that would always work for them, someone that would alert them and "beat" them whenever necessary.
Bit by bit, the kingdom shrank and lost its influence on the world. People stopped paying the traversal tax and attempted to just pass through the Isles by themselves, sometimes aided and sometimes raided by the rebel forces. The rulers did not know or did not care, for they lived in splendour still. It was then that their newest commission was finished, a ship so large and pompous that it would barely even float.
Not all wars end with a big bang, the taking of a capital, sometimes your enemy just packs up and leaves. It is said that the royals of this time did just that, giving command of their last fleet to someone they never should have trusted, to someone that would lead the fleet into their doom, forever vanishing with it. That fateful day, mists coalesced throughout the Isles and swallowed the whole fleet. No ones knows where they went and no one really cares. The rebels found nothing, only empty islands, some of them with a few manned outposts, soldiers that had not been deemed necessary, prisoners left to their own devices and fishers that had never heard of them before.
From this time, a new rule arose. They would never tolerate a king or queen anymore, they would govern themself and never accept more rule than absolutely necessary. The first Council of The Open Sea was formed out of the leaders of the rebel cause, over the years the members would change and grow apart until nothing really united them anymore. What binds them still is their love of freedom and the strong belief that you need to seize your own destiny.

When the mists are thick and seem to cover not only the sea, but the islands as well, some people claim to see ships hidden within. With strange shapes and forms, they are unlike the modern styles, they tend to keep away from any regular ship and often vanish as fast as they appear.
Whatever trap they were led into seems to still keep them bound to the Isles, forcing them to eternally watch the decline of their former kingdom into everything they have ever despised. Sometimes they take an unlucky crew, you will find their ship just floating around with no one on board, everything just as it was the moment the mists started to roll in. Sometimes the sailors will throw their loot overboard to appease the spirits, gain their favour and avoid sharing their fate.
What will you do, when you the see the last fleet of the Shifting Isles?