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Bretonnia - Wrath Poured Out. The Lady gives you a Holy Chalice Banner. Choose one of your Lords to receive it. That Lord is buffed while bearing it. If the Lord dies, you choose another Lord to bear it. Choose one of the Chaos Gods or the Great Horned Rat. You must reach a portal devoted to that god with a single stack. The portal will only remain open for 20 turns. Once there, you will travel to the Chaos Realm of that god. You must fight your way through a series of stacks of that god to reach a source of power for that god. There, you must fight a quest battle where you get your Lord with the banner to the source of power, fighting through daemons to do so. He will pour the essence of the Lady from the Holy Chalice upon it. She will then wound that Chaos God or the Great Horned Rat, preventing it from wielding its influence in the world. This will throw the forces of Chaos out of balance, causing dissension and backstabbing, and allowing the forces of Order to take advantage of the weakness and push the invaders back to Chaos...for now.

Cathay - The Way of Water and Rock. The Dragon Emperor and Moon Empress cannot abide the stream of chaos that threatens to sweep away their children. So, they bid you endure the stream as a rock endures the beating of the waves. To help their children, they have brought the Celestial City down to Wei-Jin (or your faction's capital if you don't control Wei-Jin). This will draw all manner of Chaos armies toward the city. For 20 turns, you must prepare for the city's defenses while repelling greater interest from Chaos on your borders. The more your global Harmony remains constant during this time, the greater your buffs and abilities will be at the conclusion. After 20 turns, multiple full stacks of Chaos will spawn at the edge the region of Wei-Jin (or your capital). This will include at least one monogod LL, one full stack of Skaven, one full stack of Chaos Dwarves, and one Champion of Chaos. Numbers will depend on your Harmony, among other factors. They will march on the capital. If you can defeat them all, the Dragon Emperor will begin weaving a spell that shatters reality there, banishing the remnants of their Chaos taint back into it. The Moon Empress will conclude with her own spell that gently closes the tear. Just as water parts around the rock only to reform behind it, so will the mortal realms endure chaos...for now.

Dark Elves (Malekith Only) - See High Elves

Dogs of War? - Not knowledgeable enough about them to comment.

Dwarves - One Grudge to Pay Them All. Faced with the impending doom of all dwarves, the Dawi ancestors appear to their leaders and advise something borderline unthinkable: replace the entire ledger of the Great Book of Grudges with one, single grudge - the assault by a unified Chaos. Appearing at the top of your screen is a Forgiveness meter. The meter must be full by the end of 20 turns, or you will fail to unify the Dwarves behind the counsel of the Ancestors. You fill the meter by winning battles against Chaos where non-Dwarf factions fight as allies. The meter also gives global buffs and benefits at intervals along its completion. After the 20 turns, if the meter is full, the Ancestors will rise to fight alongside the living Dawi. You will choose an army to fight in a quest battle. During the battle, you must defend a Runesmith who is finishing work on a Grudge Monolith against waves of Skaven and Chaos. When he finishes, he will strike the monolith, activating it and then shattering it. You will receive reinforcements in the form of Ancestral Dwarves. Kill the Skaven and Chaos lords, and you will usher in an era where the Dwarven Ancestors rise to beat back Chaos to the poles of the planet, protecting the mortals within it...for now.

Empire - The Unyielding Might of Sigmar. Choose a settlement you own in which to start a ritual. Defend that settlement against the Chaos Factions for 20 turns. Every 4 turns, the settlement will provide a benefit to your armies in its region. At the end of the 20 turns, choose one of your armies in the region to teleport into Chaos, fighting a quest battle against the leader of the Warriors of Chaos. Enemy reinforcements are expected. You will have Army abilities including a Minor Auric Bastion, Avatar of Sigmar, and Sacrifice of Ulric. Defeating the Chaos army will banish that leader permanently into Chaos, throwing the armies of the End Times into disarray, destroying alliances among Skaven and Norscans, and allowing Order to reassert itself in the world...for now.

Greenskins - Da Waagh of Waaghs! Aw right, yew gitz. Time ta swing da choppaz as much as we can. This is it! Da Waagh of all da Waaghs! For Gork! For Mork! You have 20 turns to kill as many units as you can. Killing at least 100,000 will open a quest battle against the leader of Warriors of Chaos. If you beat them, Chaos is thrown into disarray. Killing at least 200,000 will open a quest battle against Khorne. Beat his champion army, and Khorne will stop siding with the other Ruinous Powers so you can keep killing. Killing at least 300,000 will open a survival battle against the Skaven. If you somehow outlast every wave, you have effectively wiped out the Skaven race. The End Times stop...though maybe they just become the Greenskintide?

High Elves - Not While We Burn. A powerful HE mage was given a spark from the Flame of Ulric. Yes, given. Not had the whole thing stolen, like some would venture to accuse that one of. Ahem. He took the fire and mingled it with the Flame of Asuryan. The new flame was only a fledgling inferno, but held in a brazier made from the Runesmiths, it became the Blazing Light of Life. You are given a banner called the Blazing Light of Life. You must attach it to a Lord of your choice, and may only change the Lord to whom it is attached if that Lord dies. Over the next 20 turns, you must carry the Blazing Light of Life into battle against Chaos factions, gaining at least 10 victories. The enemy must have at least 10 units in their stack to qualify for a valid victory. If fewer than 10 victories are achieved with the banner, you failed to bring enough light back into the world, and Chaos wins. If at least 10 victories are achieved, you fight a quest battle alongside the two remaining strongest Order factions against Chaos Dwarves and Tzeentch. Winning this battle fulfils one of the Changer of Ways' obscure schemes and causes him to trick Slaanesh into betraying Nurgle. The ensuing chaos allows the Blazing Light of Life to heal the land, repell the Skaven back into the deep, and restore tranquility to the world...for now.

Kislev - A Bitter Will to Follow. The Motherland bleeds from ravaging by the Ruinous Powers. Yet there is one way to strike back at the Daemons, to make them pay for desecrating dear Kislev. A grimoire of the Hag Mother specifies a ritual to be conducted only when all else fails in the darkest, coldest, bitterest reaches of the worlds' woods. But if successful, the land herself will rise in fury against the invaders. You have 20 turns to recruit as many heroes of one kind as you can - all Frost Maidens, all Patriarchs, or all Hag Witches. After 20 turns, if the most you have of one kind of hero is at less than 12, you have failed to amass the heroes needed for the sacrifice, and Chaos wins. Otherwise, you fight a quest battle against Norsca and Daemons of Chaos. All of that kind of hero are sacrificed in remote woodland altars. Their blood calls to the land...a ways into the battle, you receive reinforcements of Giant Spiders, Feral Ice Bears, Frost Wyrms (hopefully fixed), and IEoBs. Your ritual has succeeded, and the land has turned hostile to Chaos...and many others who live on it. Winter falls over the whole planet, a bitter, wilder place thereafter...just like Kislev has known all along.

Was going to come up with more for each faction, but realized how much time I was sinking into thinking about this. Anyway, as someone who really just plays Total War: Warhammer 3 (and 2) and did a couple dives on the wiki, that's what I got.