The harrowing seems like it was always relatively powerful. Part of the frequency is due to those that sailed near it as the harrowing expanded (i.e. incident at 33 yrs. ago, then Morde attacks the following year). It seems really peculiar that nobody has learned anything about it in the 49+ years that the harrowing has been happening. Thus, it keeps extending and becoming stronger.
I - 1 Year. The Battle of Knife Straits. An armada from Bilgewater, led by Miss Fortune and Captain Gangplank – sailing together under an uneasy truce – meets the Black Mist head on. Heavy casualties sustained. Fleet scattered.
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II - 1 Year. The Drowned Anchor. One element of the Harrowing stalled. Reports of a large figure encased in a rusted diving suit seen in the heart of the fighting.

III - 1 Year. The Reaping of Smuggler’s Cove. No survivors.
IV - 3 Years. The Battle of the Serpent. A Blue Flame leviathan is goaded to the surface by a serpent caller to face the Harrowing. The Black Mist is dispersed before it reaches Bilgewater, but the giant from the depths is slain in the process.
^ this could be Nami 
V - 9 Years. The Shadow Falls. The Black Mist overcomes the defenses of the Southern Beacon, and it sinks beneath the sea. Its guiding light is now lost to Bilgewater.
VI - 14 Years. The Wild Hunt. Hecarim marshals a spectral host and lays waste to the port of Grey Harbor. Despite valiant resistance from the fishing fleet anchored there, all souls are lost. Grey Harbor remains uninhabited to this day.

VII - 17 years. The Crimson Armada. A Noxian war fleet is caught in the Black Mist. Over 30 vessels lost. No survivors.
- Perhaps more Noxian lore to come out of this.
VIII - 21 Years. The Death of the Conqueror. The mighty galleon Conqueror is lost to the depths, along with 30 battleships. Some have claimed to have seen the Conqueror riding the Black Mist in subsequent Harrowings.
- Potential for the Captain of this ship/fleet to be GPs or MFs teacher/predecessor.
IX - 32 years. The Battle of Port Mourn. The revenant Mordekaiser leads the Harrowing on a night of slaughter, and Port Mourn falls to darkness. It is said that specters now haunt its empty streets.

X - 33 years. The Flower of Ionia. An Ionian fleet of unknown size lost to the Harrowing.
- Hope to see more back story of Ionia with appearances by Irelia's father.
XI - 37 years. The Maelstrom. The frigate Light-Bringer’s Bane is swallowed by the sea while sailing to outrun the Black Mist.
XII - 49 years. The Shroud. Up until half a century ago, this was the furthest extent of any known Harrowing.
- Interested to see what the boundaries/reasons were that the kept the shroud in check and why they wore down.