Fourth Great Maelstrom is a term used by Wraiths to describe a violent superstorm that happened within the Shadowlands, that was triggered when the First World War and the Spanish flu struck Europe, between 1916-1929.
The Fourth Great Maelstrom began on July 1, 1916, triggered by the enormous loss of life at the Somme River battlefield. A massive nihil opened there and remained open until the storm ended; other nihils opened around Europe, especially where other major battles had taken place. Spectres flooded forth from the nihils, stirring the Tempest into a frenzy. Almost immediately, the Spectres conquered the Necropolis at Florence, creating their own Citadel and using the captured wraiths and relics to make enormous pyres. From here, they spent the rest of the Maelstrom attacking the other major European Necropoli.
The storm became one of the most difficult in wraithly history to navigate; all but the heartiest Harbingers were afraid to navigate even well-known Byways. Trade between the continents shut down, Stygia found itself cut off from the rest of the Shadowlands, and wraiths found that even their Fetters that rested on other continents were unreachable; any wraith that lost all of the Fetters on the continent they were trapped on went into a Harrowing as if they had no Fetters at all.
The Dark Kingdom of Ivory took advantage of the Maelstrom to cleanse themselves of the invading Europeans; white and Indian soldiers who went to Africa to fight for the colonies and died there were forced into the Maelstrom by the Ivory Queen. Some estimate that only one wraith for every ten sent out of the Bush of Ghosts made it through the storm to Stygia or a Necropolis, and even then, the Indian wraiths wanted to continue on through the storm to their homeland of Swar. Most Necropoli, already under a great strain, made as much room as they could for these pitiful, ragged souls.
Most of the Necropoli withstood the storm; several successful campaigns were made against the Spectres by the Legion divisions not wrapped up in the struggle against the Smiling Lord. The dead soldiers from the War to End All Wars proved to be a great asset by bringing relic planes with them to fight above the storm. By 1929, the nihil at the Somme closed and the Fourth Great Maelstrom ceased to be.