The Forming of the Devil's Shroud
The Devil's Shroud, the name given to the impenetrable wall of fog that surrounds the Sea of Thieves, has existed since prior to Ancient times. How the Shroud came into being is currently unknown.
The Ancients used artefacts like beacons to protect their homes and places of worship from the Shroud's effects or created relics for safe passage into the fog.
As the Devil's Shroud formed, cutting off access from the outside world, the Sea of Thieves is now trapped within this bank of impenetrable fog.