Sort of inspired by A Shattered Road on AH.com
Basically Tywin Lannister intervenes on the trident, and through a mixture of surprise and masterfully planned deception destroys both armies-royalist and rebel. With the aim of being crowned king. Having put all the lords on the trident to the sword. He sacks KL, and plans to marry Rhaella,or otherwise forcibly interject his blood into the Targaryen dynasty, if not rule through the sword alone.
He manages to complete the sack, before word reaches Lord Tyrell, seeing the whole order of Westeros collapsing-Lord Tyrell marches away from Storm's End, with Stannis and Renly having finally died of starvation(Davos sunk in the sea), and meets Lord Tywin at King's Landing, a brutal battle ensues just as the Tyrells storm the red keep, having slain Tywin, Kevan, and many other westermen, the wildfire goes off. Killing everyone there.
At this point Westeros breaks down, entirely into various warlords with House Lannister and House Tyrell being ruled by boys(Willas and Tyrion) with older regency councils, the north is ruled by a young Benjen, and the Riverlands are in theory ruled by Catelyn Stark, but in truth is a mess of petty kingdoms and roving bandits.
The stormlands at this point are literally in famine, the Tyrell host had confiscated all the food available on its march north. And anarchy. The Iron islands have erupted into a civil war between the fruits of Quellon's loins, with Balon, Euron and Victarion all wishing to pursue different avenues of conquest, and rallying their own supporters.
Doran sits alone, Dorne aflame with rebellion and cries of vengeance, against the Lannisters, against the Targaryens, against the Tyrells even. Seeing Highgarden overrun by a Florent-Tarly-Rowan bloc, and Willas Tyrell killed, Doran gives Oberyn the go ahead to invade the Reach, and another army under Lord Yronwood to invade the stormlands, the plan? Ostensibly territorial acquisition, the reality? Blind rage with a lack of a clear focal point.
And who will be the savior of this Westerosi Age of Warlordism? Why Viserys Targaryen of course! Married to Daenerys at his mother's last command, Viserys manages to have a large coterie of fled lords, bureaucrats, and knights around him-he manages to gain the support of the GC, and also negotiates a deal with the three daughters and Braavos-granting them vast economic concessions among other things, in return for sellswords and sellsails. He returns to Westeros 20 years after the fall of KL, but unfortunately must literally reconquer the country-owing to the actions of Lord Tywin and Lord Tyrell, the process of gaining acquiesce and consent to rule(however much that means something in a feudal society) was replaced with a "live by the sword, rule by the sword" unvarnished savagery, or more artfully-"to the strongest" warring lords do not even bother with fealty, its every man for himself, and anyone with the biggest army can lay claim to some sort of crown. Often lords are killed by their own retainers who are themselves cut down-feudal obligations basically disintegrate. As law and custom vanish into rule by swordpoint-literally.
So Viserys must not only reconquer what of Westeros he can, but also restore the social compact. When its nobility have abandoned all law and justice for a jungle like environment of anarchy and neverending slaughter.
This he must do quickly, as around 301 AC, a large blast sounds in the north which shakes the earth to its foundations-the wall falls down from end to end, and is but flat snow. And the dead rise from the graves, ditches and pits-with the Others coming south full bore.
