The Aftermath of Mount Ator
The closing of the Elven Wars left the political map of Auridon in pieces. The Triumvirate had won, but the working at Tanzelwil had drained the Sorcerer-Kings of much of what they had drawn from the Heart, and the long campaign against the Princes had ended with Nerevar dead at Alqualonde and the Chimer host transformed into the Dunmer. The Golden Covenant had collapsed amid the chaos created by the natural disasters caused by the conflict. The Bosmer withdrew into the forests in the centre of the island. The Dwemer withdrew underground and were not seen again above ground in Auridon for millenia.
The geography itself had changed. The land bridge that had once joined Summerset to the rest of Tamriel was gone, the seas had risen between, and the currents of those seas would not settle again. The realm of Summerset, which until the war had ruled Auridon as its eastern region, found itself separated from its eastern province by waters no fleet could reliably cross. Within Auridon, the Altmer of the south and the now-Dunmer of the north were left to reorganise themselves alone.
The Realm of Ascalon
The cities of the north were Chimer foundations from their origin, before any cultural distinction between Aldmer and Chimer had hardened. By the beginning of the Elven Wars they were under the nominal authority of Mournhold, the capital of Summerset, but just like all the other cities of the Altmer dominion they had a high level of independence. With the closing of the seas after Mount Ator, that nominal authority became impossible to enforce, and within two decades the cities had ceased to acknowledge any external crown. They formed a federation among themselves and called it Ascalon.
The three Sorcerer-Kings remained on Auridon for the entire Merethic Era and ruled the realm directly, each taking residence in one of the three major cities.
Vivec claimed Balmora, the oldest of the three. The site had been swamped by the rains of the late Daedric Era and was uninhabitable when the war ended; Vivec drained the marshes and rebuilt the city as the religious centre of the Tribunal doctrine. The men who came to Auridon in the later Merethic Era called the place Firsthold, on the reasoning that it was the oldest settlement on the island, and the name has stuck in human usage. The modern town built atop the ruins of the original is called Ba-Xo-Cil.
Sotha Sil claimed Tel-Aruhn, a city on the western coast, and from it directed an excavation that drove north into the Jerall Mountains. His laboratories and furnaces were built into the mountain rather than in the city itself, and Tel-Aruhn served as their surface gate. The modern name of the city is Tong-Rao.
Almalexia claimed Sadril Mora at the southern edge of the realm. Its central position on the island made it the natural seat for diplomacy with the Altmer of Ulumbra to the south and the Bosmer of the Dragon Plains to the west, and Almalexia worked from it for most of the era to maintain communication with both. Some chroniclers hold that diplomacy was not her only reason for choosing Sadril Mora, and that she remained there because it was the city closest to Alqualonde, the resting place of her late husband. The city became the bureaucratic centre of Ascalon. Sadril Mora is the city that, many thousands of years later, would be taken by the men that escaped Darkmoor and refounded as Eglarest.
The Realm of Ulumbra
The Altmer of the southern half of Auridon were left in a worse position than their Dunmer neighbours. Several of the great cities of the south had been depopulated and the surviving nobility had lost both its chain of command to Alinor and its faith in it. Two cities became the centre of what was rebuilt.
The first was Falas, on the southwestern coast, which had been spared the worst of the fighting and kept its walls, its archives, and a functioning council. After Mount Ator it absorbed refugees from the ruined inland cities and doubled its population within a generation. The second was Lindon, an older Aldmeri foundation whose Daedric Era importance had been chiefly religious. Lindon had hosted a major shrine to Azura, and after the defeat of the Old Tribunal the shrine fell out of use and the city lost most of its standing. Lindon would, much later, be colonised by humans and refounded as Kortirion.
The realm that Falas and Lindon agreed to form was called Ulumbra, from an old Aldmeri word meaning the shadow or the place after dusk, and the name was chosen with full awareness of its melancholy. Ulumbra's founders did not pretend to be the heirs of Alinor. They held that the Altmer of Auridon had been severed from their homeland by the same powers that had ended the war, and that to claim continuity with a realm they could no longer reach would be to claim a thing that was no longer true.
Lindon was named the capital, but the title was ceremonial in the Altmer fashion: Ulumbra was a federation of semi-independent city-states, after the pattern of Alinor itself, and Lindon's role was symbolic rather than administrative. Falas held the greater practical weight. It became the centre of Altmer research into the crossing of the sundered seas, for the Altmer of Auridon never ceased to long for escape from the island after their defeat by the Dunmer, and the question of how to reach the mainland became the chief intellectual project of the realm.
Ulumbra never flourished. It maintained itself, slowly, through the Merethic Era, and kept a peaceful relationship with Ascalon for the whole of its existence until it faded with the emigration of Altmer in the late Merethic Era. The peace was sustained in large part by the exchange of research between Falas and Tel-Aruhn.
The Withdrawal of the Dwemer
The Dwemer of Auridon left the surface in the years after Mount Ator and did not return. Their patron Mehrunes Dagon had been driven from the field by Nerevar, the alliance with the Old Tribunal had ended with the Princes' banishment, and the surface mer of the island had no further interest in their company. The Dwemer themselves regarded the matter as settled. They retreated into the underground freeholds they had been building since the opening of the Daedric Era, sealed the surface entrances, and made no attempt to assert any claim to the lands above.
What contact remained was minimal and hostile. The Dwemer continued to mine deep and greedely and occasionally surfaced to take materials they could not produce below or capture slaves. They did not negotiate, did not trade openly, and did not respond to embassies sent from either Ascalon or Ulumbra. By the second century of the Merethic Era, the Dwemer had become a completely isolated civilisation.
Around the year 3E600 their excavations broke into Agartha, the deep realm beneath the world's roots, and there they found etherium, a metal whose properties exceeded anything they had previously worked. The discovery transformed Dwemer civilisation. Their technology, which had already outpaced that of the surface mer, advanced at a rate the surface could not follow, and the gap between the two civilisations widened into something it had no plausible means of closing.
The Dispersion of the Bosmer
The Bosmer who had fought beside the Covenant did not regroup. In the closing months of the war the Khajiit host had broken their lines, the engagements between Khajiit and Bosmer had been the most savage of the conflict, and what survived of the Bosmer companies dissolved into the forests of central Auridon rather than rally to a defeated banner. They made no attempt to claim any of the cities the Covenant had abandoned.
The forests they entered covered most of the central highlands of the island, the Dragon Plains of later cartography, and they extended south as far as the Siriolnir. Whether they ever organised formally is unknown. Some chroniclers hold that they did, and that the Bosmer of the Merethic raised cities of their own somewhere in the deep wood, hidden from outside eyes by the forest itself and by the discretion of their builders. No such city has ever been found, but the tradition is old and consistent enough that it cannot be dismissed outright.
If the cities existed, they are believed to have fallen to ruin with the rise of the Lich-King, whose virulent necromantic plague of 3E3000 started withing the bosmer forests themselves.