Storm of Chaos
Storm of Chaos was a big worldwide gaming event for the 6th edition of Warhammer Fantasy, fleshed out as the biggest invasion yet by the Hordes of Chaos under the guidance of Archaon against the Empire, with various other factions reacting.
Things didn't go as planned, with both Hordes of Chaos and GeeDubs' balancing teams getting embarrassed left right and center by more or less everyone. The event was a failure, with awful fluff that resulted in the status quo being painfully reverted to by having Grimgor Ironhide ambush Archaon, coldcock him into unconsciousness with a headbutt, then bugger off content that he was the hardest.
It has since been retconned out of existence, though its spirit has been reborn in the form of The End Times for 8e.
New Armies
Since this was the time when Warhammer Fantasy was pretty awesome, the Storm of Chaos had a lot of new armies that built on existing armies, allowing a player to represent more unique forces born for the event.
Archaon's Horde: A variant of the Hordes of Chaos representing the humans directly under Archaon's command. It introduced the new units Flayerkin (now-forgotten mutant skirmishers and seige-breakers) and the Hellcannon (which has since become a mainstay for Warriors of Chaos).
Daemonic Legions: This was the era of Hordes of Chaos, so whilst pure-Daemon armies could be done, they weren't very good. The Daemonic Legion, which may have given rise to the eventual Chaos Daemons armies for Fantasy and 40K, made them more viable, with some rule-tweaking and the addition of Daemonic Heralds, Mounted Daemonettes (this was the era where classic Steeds of Slaanesh were made a new kind of daemonic steed, unique to this unit, and the name Steed of Slaanesh was given to new snake-daemon beasts), skirmisher-packs of Flamers, Daemonic Cavalry and Daemonic Chariots.
Middenlanders: A variant of the Empire army that focused on the Ulric-worshipping forces of Middenland. Complete with its own armory of magical items, disallowed Empire items, and new units; the White Wolf Knights' Grand Master, the Seneschals of the White Wolf, the Priests of Ulric, the Teutogen Guard, the Warriors of Ulric, the Wolf-Kin and the Hunting Hounds.
Grimgor's 'Ardboyz: A variant of Waaagh! Grimgor Ironhide showing Grimgor's restructured horde after his loss to Vardek Crom. No limit to the Black Orc units, but can't take any Goblin units besides Rock Lobbers and Spear Chukkas, and with the new Effigy of Gork unit.
Slayers of Karak Kadrin: What better opportunity for Slayers seeking an honorable death in battle than to throw themselves against the hordes of Archaon? This lets you field an entire army of Dwarven Slayers, with some new special rules to make them more viable and two new units; the Doomseekers (Slayers who wield axeheads welded to chains wrapped around their wrists, which they use like flails) and the Brotherhood of Grimnir (basically Troll Slayers with Magic Resistance 1 for +3 points per model, and using up a Special slot).
The Cult of Slaanesh: Slaanesh worshipping Dark Elves who feel the time is right to come out and play. Can only have Morathi as a special character, various Dark Elf characters/units can be upgraded to have the mark of Slaanesh, and they can include a Keeper of Secrets, Aspiring Champions of Chaos, Marauders of Chaos, Warriors of Chaos, Knights of Chaos, Fiends of Slaanesh, and Daemonettes, infantry or mounted. They also have the unique Druchii Anointed and Devoted of Slaanesh units, and some unique magical items.
The Army of Sylvania: Variant Vampire Counts army reflecting the Von Carsteins of Sylvania rising to their nation's defense.
Errantry War: A Bretonnia variant army representing their efforts at crusading against Archaon's hordes.
Clan Eshin: An extra sneaky Skaven variant army.
Sea Patrol: A High Elves variant army focused on the naval forces, complete with new spells and a sea-going monster called a Merwyrm as a unit choice.
Characters
Several new characters, or re-statted old ones, appeared in the Storm of Chaos.
Valten: Special character for the Empire.
Vardek Crom: Special character for the Hordes of Chaos, especially Archaon's Horde.
Be'lakor: In his first ever appearance as a character, he is an auto-general character for a Daemonic Legion.
Boris Todbringer: The Elector Count of Middenland.
Ar-Ulric Emil Valgeir: The High Priest of the Cult of Ulric.
Borgut Facebeater: One of Grimgor's toughest and most loyal followers.
Ungrim Ironfist: The Slayer King of Karak Kadrin.
Garagrim Ironfist: The War-mourner of Karak Kadrin.
Ruglud's Armoured Orcs: A Dogs of War unit consisting of crossbow-wielding orcs in heavy armor.
Malakai Makaisson's Goblin-Hewer: A Dogs of War unit consisting of a rapid-fire automated axe-throwing machine, created by a character from the Gotrek & Felix stories, who appears to control the machine.
Long Drong's Slayer Pirates: A Dogs of War unit of mercenary Slayers turned pistol-wielding pirates.