Aragorn, King of Dale, defeats Witch-Kings of Gondor and claims Mordor for the elves.

Here's the situation in around 1200 of the Second Age. The Elven realms are growing ever more fair: Lindon is blossoming in the north-west (around Mithlond), and there's a lively trade around Khazad-Dum, Eregion, and Lorien. At this time, a powerful and beautiful stranger calling himself Annatar, 'the Lord of Gifts', comes out of the east, offering aid to the elves. Gil-Galad and Elrond bar the gates of Mithlond, refusing him entry.
At Ost-in-Edhil, Celebrimbor, Galadriel, and Celeborn of Eregion... do exactly the same.
Sauron (yes, it was him) returns east, his plan to corrupt the Noldorin kingdoms a failure. He managed to gain some influence in the other elvish realms - Eryn Galen, for instance - but they are Sindar and Nandor, and Morgoth's heir has no interest in trees. So he holes up in Mordor, building the great fortress of Barad-Dur, and preparing.
In Eregion, Celebrimbor's folk continue their research into 'magic' rings, trying to stay the ravages of time on their realm. With no Annatar to drive them out, Galadriel and Celeborn remain in the Noldorin realm, lending their skill. Over the centuries, the art of ring-making is perfected: from minor rings the smiths progress to Rings of Power which can preserve mortals against death and heighten the bearer's innate powers, and then, finally, to a single Great Ring which can sustain an entire nation against decay.
The Rings are gifted to Eregion's allies: to Durin of Khazad-Dum, to Gil-Galad, Elrond, and Cirdan, to Amdir of Laurelindorenan, even to mortal folk living on their borders. But the Great Ring is kept by Celebrimbor, and worn by him alone.
Somewhere around S.A. 1800, Sauron completes the millennial labour that is the construction of Barad-Dur. He openly declares himself as Morgoth's Heir - but the time is not yet ripe for him to strike. His agents attempt to sow discord among his enemies, while the Dark Lord waits for an opportunity, and a pretext.
By around 1900, he finds his opening. Hordes of orcs swarm across Middle-earth, advancing through what we know as the Gap of Rohan and entering Eregion in war. They are aided by the mortal men of the forests, who rise up against the Numenoreans oppressing them, and swear fealty to Mordor.
The Ringbearers gather in Ost-in-Edhil, bending all their might against the Dark Lord. They have no hope of victory - Gil-Galad's overtures to Numenor have been turned down flat by Tar-Ciryatan. All they can do is prolonge the siege as much as possible, to buy time for their folk to flee.
Yet Numenor comes.
Not out of friendship with the elves - Tar-Ciryatan has long disliked their folk, and in his court speaks openly against the Ban of the Valar - but to protect their own interests, the logging grounds and the haven at Lond Daer. The Numenorean Grand Fleet sails up the Greyflood and shatters the years-long siege of Ost-in-Edhil. Sauron's army is in tatters, and the Dark Lord flees back to the east. The Free Peoples have their victory, and Eregion stands.
Yet the Shadow is still spreading over Numenor. Men of Westernesse continue their mass deforestation of the lands immediately south of Eregion, and their cruelty to the native mortals causes sporadic uprisings. Anti-elven sentiment rises in Numenor, until eventually, hundreds of years after the defeat of Sauron, the inevitable occurs: war.
The Numenorean navy again sails up the Greyflood in battle. Ost-in-Edhil is caught unawares. There is to be no second Siege. Numenorean fire meets elven art - and it is the elves who are broken.
As Eregion falls under the iron fist of Numenor, a second fleet blockades Lindon. The soldiers do not attack, but their message is clear - any attempt to relieve Eregion will be met with death.
The great folk of Eregion flee through Moria. They are not welcomed in Laurelindorenan, but continue northwards to found a new settlement around the upper Anduin. In their former home, Numenor gathers up all the magic artefacts it can find - including many of the Rings of Power. Tar-Vanimelde, Ruling Queen of Numenor, will never yield the sceptre: she and her husband will reign forever, and her armies will be led by immortal generals.
The centuries pass. The situation in Numenor grows worse and worse. Lindon, under threat of invasion, slowly empties, ceding the lands west of the Misty Mountains to Men. In the east, Sauron's armies grow once more in power. In former Eregion - and further south, around Anduin's mouth and down in Umbar - the Numenoreans build havens, claim territories, and take slaves. A clash is inevitable, somewhere, between some parties.
Sauron moves first. Jealous of Numenorean power, he seizes their lesser forts up Anduin - and in the north, Gil-Galad and Celebrimbor, influences by Sauron's agents, declare their own war. An alliance of elves and dwarves storms out of Khazad-Dum and down into Eregion.
But all involved have underestimated Numenor. Ar-Abârzîr (Tar-Vanimelde's Adunaic name, for she no longer allows the High Elven tongue to be spoken) storms into Middle-earth. Her armies are unstoppable, and her invisible spies, bearers of Rings of Power, can penetrate any camp not staffed by Eldar of Valinor.
The attacks fail, and worse: the leaders of both agressors are taken captive. Sauron, Gil-Galad, Celebrimbor, Galadriel, and Durin IV are borne in chains to Numenor. Khazad-Dum is laid waste, Numenorean soldiers cross the High Pass, and the Last Alliance of Elves and Dwarves are forced to flee beyond Greenwood, to the Long Lake and Lonely Mountain. Immortal Numenorean generals are set in place over the various fallen realms - including Mordor.
In Numenor, Ar-Abârzîr's captives are paraded before the people. It amuses the Witch-Queen of Westernesse to listen to their bargains and pleas - and that, of course, suits Sauron just fine. It isn't long (a couple of hundred years) before he is able to convince the queen to listen to him.
In whispers he tells her of Celebrimbor's Ring - the Ring that can give her back her beauty, rescue her from her shadow-life, but let her keep her powers - nay, even make them greater than before. Swayed, Ar-Abârzîr takes the Great Ring from Celebrimbor - and, when it does not do as Sauron promised, has the Elven smith tortured to death over several years, trying to extract the secret from him.
It takes Sauron longer to bring about his true goal. Slowly, slowly, he persuades the Queen that the Valar are responsible for her half-alive state - that if she can conquer the Blessed Realm as she has Middle-earth, she will have the life she craves. Centuries pass as Ar-Abârzîr prepares her armies, develops her technology (using captured Elven smiths), and lays her plans.
With the elves no longer an active enemy in Middle-earth, the Faithful of Elendil's line (for Elendil himself is long-dead by this time) have something of a resurgance. When the Witch-Queen finally launches her attack, they are ready - not to sink her fleet, nor to attempt to sieze the throne, but to rescue the imprisoned Elvish leaders and flee to Middle-earth.
The Faithful's plan succeeds. So does Sauron's. When the Witch-Queen sets foot in Aman, the Valar lay down their guardianship of Arda. The world is made round. The only escapees from the Downfall of Numenor are the small ships of the Faithful, bearing their precious elven cargo.
But there is more to the Numenorean world than just the isle. Armies and immortals still watch over Middle-earth, and they are not about to bow to Faithful traitors.
Here is Middle-earth after the fall of Numenor:

The Faithful land in abandoned Lindon and take the treacherous ways north of the mountains to Erebor. The Numenorean havens unite to form a Kingdom in Exile - but an Adunaic-speaking one that is hostile to the elves. Sauron's spirit cannot return to occupied Mordor - he flees further east. Since the fall of Numenor and the loss of his body has weakened him to the point where he will take centuries to recover, the Downfall is accounted the end of the Second Age.
The Third Age will have to be sketched in broad strokes. A mingled civilisation of elves, men, and dwarves is built in northern Rhovanion, taking in the Greenwood and the Iron Hills, and extending down as far as Dorwinion on the Sea of Rhun. They face incursions from the east as Sauron rebuilds, and an implacable enemy to the south.
The Numenoreans build a nation across the south. With Eregion's trees long gone, and the 'New Hollin' settlement constantly raided by the folk of Erebor, they pull back to the Greyflood. Calenardhon (=Rohan) and Dunland make up one facet of the kingdom; southern Gondor and the lands down to Umbar make the second; Ithilien and Mordor (when it's not held by the orcs, which happens fairly regularly) make the third. The last three Numenorean immortals become three kings, sometimes firm allies with each other, sometimes in open war. They will always unite to counter the elves, Sauron, or the Haradrim, however.

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