There have been five Eras by the time of Skyrim since Convention. The first is called the Merethic Era, and by magical dating of Ada-Mantia mages ascertain this was about 2500 years long. IIRC it's also numbered backwards like BC or BCE is in real life. "Mer" is the proper term for elves, Merethic was when the proto-elves, the Aldmer, and their various offshoot races ruled most of Tamriel, and the only empires of Man were in Skyrim and Atmora, where they were actually ruled by and worshiped dragons, parts and avatars of the great Aka-Tusk (which the Aldmer worshiped under the name Auri-El, while the Nedes and Nords worshiped it under many names, one of the chiefest being Alduin, who would eat the world so a new one could be born when its years were spent). The dragons also recognise "father Akatosh" as an even higher authority than Alduin, meaning the Nords likely worshiped him as well. In this time most of their gods were totems, though. we don't know what all of them were for absolute certain (though we have good guesses on all those we see in Skyrim). We do know, for instance, that Lorkhan was the Snake, Aka-Tusk the Dragon and Kyne, the wife of both of them, the Hawk. Schisms and poor management in this cult by the dragons led to much strangeness, like the Dragon Priests on Skyrim turning corrupt while those on Atmora were still admired, which eventually turned mankind against the dragons themselves in an event called the Dragon Wars, in which the dragons' authority was questioned, and, dominance being in their nature as part of Aka-Tusk, they struck back until eventually Alduin was banished by exploiting an Elder Scroll, which are mysterious indestructible items which seem to be pieces of Anu's mind, or something similar.

But away from mankind, there was strangeness in Aldmeris (not an actual continent like Atmora, despite later Altmer (high elf) belief, just the lands of the Aldmer around southwest Tamriel) as well. The Summerset Isles were apparently seen as a paradise after Aldmeri explorers scouted the rest of Tamriel and found it lacking. Being exiled from the Isles was seen as a penalty worse than a death sentence to the Aldmer (and later the early Altmer) and few desired to leave. Some that did seemingly leave of their own accord included the Direnni clan, who became prolific mages and lived an easy life with Nedic (proto-man) concubines in what would become High rock neighboring Skyrim. The Direnni produced so many offspring with men that it produced a new half-bred race called the Manmer or Bretons, who appear mostly human but have the elves' easy grasp of magic and talent for academia. Another group to leave (probably, though they were heretics who worshiped some Daedra over even the Aedra and being exiled is a distinct possibility) willingly were the Velothi (for they followed the prophet Veloth, who believed the elves had become decadent), who took on the name of Chimer when they arrived far north in the mountains they would also call Velothi, in the land of Dwemeris, which would come to be called Resdayn when it was under joint rule of the Chimer and Dwemer, then eventually Morrowind. Chimer means "Northern elves." Dwemer means "deep elves," likely because their cities spanned far underground.

On the other hand the Dwemer obviously left before the Chimer did, and were the most heretical fuckers of the lot. They believed, essentially, that despite the fact that all of the gods clearly existed, and despite the fact that reality existed, all of it was also false. They're often compared to fedora atheists because they had a seemingly very smug attitude about their disbelief in reality itself. They were also easily the most intelligent race on Nirn until they disappeared, but more on that in a bit. The Chimer and Dwemer had many disputes with each other and the Nords over land, but eventually the Dwemer and Chimer found peace because some of their leaders had become close friends with each other, and united temporarily against the Nords. One prolific Dwemer clan also left Dwemeris, throwing their mighty hammer Volendrung and following it to where it landed to settle down. It landed in a place they called Volenfell, or Hammerfell, though during their migration there many families split off and started clans all over Skyrim, their cities appearing small on the surface but branching out underground.

In the Heartland were the Ayleids, or Wild Elves, and judging by their architecture the Falmer, Snow Elves, of Skyrim were originally Ayleids. Why they left the Isles is also a mystery, exile being rather likely as they had sometimes strange religious ideals (relative to the Aldmer). Either way, the Ayleids found nothing more glorious than magic and light, and harvested pieces of the Magna-Ge that fell through the stars to turn into magical artefacts.

The Falmer inhabited Skyrim before the Nords did, while most Nordic/Nedic holdings were on Atmora. When Nordic colonies settled on Skyrim they were at first friendly in their dealings with the elves, but eventually, probably owing to concerns of population (men breed faster than elves in general, and Falmer were haughty), the Falmer attempted to cull the human population, which was the first stone tossed in what would eventually become an immense racial war with genocide on both sides and probably the first real echo of the conflict of Lorkhan and Aka. Either way, the Nords fought back until it escalated to the sacking of the first Nordic city on Skyrim, Saarthal, where the Nords had found a strange artefact associated with Magnus. They sent word back to Atmora that the elves sacked Saarthal and a great hero and Dragon Priest of Atmora, Ysgramor, sailed to Skyrim with five hundred Nord heroes, his Companions, and they swept across the land slaughtering elves viciously. The Falmer that survived did so by seeking the protection of the powerful and reclusive Dwemer there, who greeted them with open arms on the condition that they be fed a fungus the Dwemer grew underground. The fungus contained a toxin to blind the Falmer and the Dwemer used their blind and broken elven cousins as slave labour and subjects of scientific experimentation. And so the Falmer society was wiped out and they became more like a race of goblins.

Meanwhile the Betmer inhabited southern Tamriel and might not have been direct descendants of the Aldmer at all, rather being close cousins. The south of Tamriel was all dense forest at the time and the Betmer were degenerate Ehlnofey who never managed to solidify their form, so they would shapeshift often uncontrollably into different monstrous forms. Azura, a Daedric Prince who takes special interest in the past and future and the well-being of mortals (which doesn't mean she's entirely benevolent, though she is generally nurturing to those that do not cross her), wished to create a race to prevent a cataclysm much further down the line by maintaining the stasis of Lorkhan, whose body was the moons, so she took some of the Betmer, who had started worshiping the Earth-Bone Yffre, who had become the forest, and gave them catlike forms and a kind of ancestral memory-magic where they absorbed memories through their mothers' milk, and the mission to climb up the sky to the moons if they were ever set off course and correct them. To deal with their shapeshifting she bound their form to the lunar lattice, the phases of the moons, and allowed them before birth to choose when to be born so they could choose their shape in life. These shapes range from magically powerful housecats to tigers the size of elephants to bipedal catmen to somewhat catlike elves and many things in between. These were the Khajiit of the Southeast (but not Black Marsh or Argonia, where the Hist lived in its toxic swamp).

According to the Khajiit it was the spirit of Nirn herself that blasted that part of the forest into desert, the land of Anequina (half of modern Elsweyr) to punish Azura and the Khajiit for altering her children (and being mutants of her children). Either way, Yffre took the remainder of the forest-dwelling Betmer and gave them the power to sustain their existing shapes more easily, becoming Bosmer, though they also have the power to perform a massive ritual that transforms entire groups into monsters in times of desperate need to hunt down and destroy threats to the forests of Valenwood.

Throughout all of this, humans somehow found their way to the Heartland, or Cyrodiil, and became Ayleid slaves. It's unclear whether this was in retaliation for what happened to the Falmer or simply Ayleid nature, and it's unclear what the slaves' quality of life was like (accounts state they were horribly and gruesomely tortured and killed for amusement, but those accounts come from the Alessian Order, the first Empress's religious order of flagrant pro-human propagandists, and not entirely unlike the Spanish Inquisition), but either way it was bad enough for Kyne, wife to Lorkhan and Aka-Tusk, to answer one slave (Alessia)'s prayers and send her one of the Ayleids' own soulgems, the Chim-el Adabal (or Royal Godstone of the Knight/Godstone of the Starlit Knight, if I have my Ehlnofex correct, or literally Starlight-knight/Royalty-knight/Splendour-knight Godstone), a red diamond soaked in elven blood from ritual sacrifices performed at a Tower they built in reflection of Ada-Mantia. Kyne had blessed the stone and when Alessia held it its facets unfolded until the small diamond became a man-size half-Ada android, built by the Magna-Ge, which would bear many names, the most common and obvious being Pelin-el, meaning "Star-made Knight," since he was constructed by the Star Orphans. Later this name would be corrupted into Pelinal, meaning "Glorious Knight." He was also one of the early Shezarrines, and Lorkhan acted through him.

Pelinal was a being of immense power and he chewed through elves and sometimes the land itself, turning the landscape into void in a blind madness until Alessia had to pray to the gods to calm him. The Ayleids worshiped a fallen Star named Meridia, who had been cast out of Magna-Ge society for being royalty but consorting with lesser castes after trying and failing to prevent two separate cataclysms outside of Mundus, and they prayed to her for a way to counter Pelinal, and one of their great sorcerer-kings, Umaril, was granted the blood of a Daedra, who Meridia became one of presumably to aid the Ayleids, meaning he was far more difficult to kill and even if he was slain he would only return to the waters of the realm she made, and was also granted an army of Aurorans, the lesser Daedra Meridia created in the image of light to serve her in her realm.

Umaril managed to come closer than anyone before to slaying Pelinal but in the end Pelinal was still the better knight, and Umaril was killed. Umaril's watching allies, the other kings of the Ayleids, all attacked Pelinal in his weariness after the battle, however, and managed not only to slay him but to pull him into eight parts, scattering his body across Cyrodiil, while the Chim-el Adabal which had remained his Heart found its way back into Alessia's possession.

Being an android and not a being of flesh and blood, Pelinal was with Alessia on her deathbed much later (or more likely only his head, which continued speaking to one of the Cyro-Nords' heroes after he was apparently cut into pieces), but in the meantime the damage to the Ayleids was too great and despite their small victory the Alessians won the war. Alessia proved a kind, wise and forgiving ruler, and accepted those Ayleids that didn't simply flee to Valenwood into her new Empire and even her court, and set about rewriting the Nordic religion into her own where the Aedra were worshiped as Divines under culturally neutral names, so neither the Nords nor the Elves could take offence to it. She made a deal with Aka-Tusk, who she named Akatosh, or Dragon-Father in the old tongue (if I have my Ehlnofex straight), to create a barrier around Nirn to keep the Magna-Ge and Daedra from manifesting there without a special summons or creating stable gateways to their realm on Nirn with malicious intent. He and each of the other seven Aedric Divines contributed a small coloured diamond, which were set around the Chim-el Adabal in a necklace, becoming the Amulet of Kings (with the Red Diamond itself representing the god Shezar, or Shor, another name for Lorkhan which the new Imperial race kept low-key around the elves to avoid fighting, as the elves loathe Lorkhan and consider his trickery to be the reason they aren't gods themselves and must all eventually die). Lastly Akatosh made Alessia a Dovakhiin, or Dragonborn, the first of the Dragonborn Emperors though not the first Dragonborn ever. With her Dragonblood, the Amulet and the Voice, Alessia could light the Dragonfires in the Ayleids' Tower where they used to entomb ancestors and perform sacrifices, powering the barrier Akatosh created. This was called the Covenant- As long as an Emperor chosen by Aka-Tusk wore the Amulet of Kings, Tamriel would be mostly safe and orderly. And so the first true Tamrielic Empire of Man was formed (outside of the smaller and shorter lived Empire in Skyrim begun by King Harald, whom I believe was descended from Ysgramor).

That Empire lasted about a thousand years, during which time a lot happened. In High Rock it was generic political intrigue and constant shifts in power and religion. In Resdayn something really significant happened, though. After allying against the Nords, the Dunmer discovered by way of one of their King, Nerevar's, close friends and advisors, Dagoth, that the Dwemer were hiding an artefact in Red Mountain on Vardenfell (the Dwemer word for "Heartfall," the island where the Heart of Lorkhan was entombed by the Mountain). While this was confirmed Dagoth went to the Nordic High King, Wulfharth, a Shezarrine, and told him that while they had been enemies before they now had a common goal because Shor's Heart had been found in the possession of the Dwemer, and that the Nords could have it if they fought alongside the Dunmer this time. Wulfharth was deeply racist toward elves but agreed nonetheless because the stakes were so high, and he was willing to believe an elf would happily stab its allies in the back for the hell of it, leading to one of the cooler scenes in TES lore. The Khajiit, sensing something of import to Lorkhan at play, sent many of their heroes, including a great sapient battle-tiger named Dro'Zira, to fight in the battle as well, and Wulfharth, an ancient lich who couldn't die but had overaged himself until he turned to ashes using his Voice (lifting a curse from the Nords that made them all children), ended up riding a giant tiger into battle against armies of Dwemer and their steam-powered robots alongside the warriors and battlemages of the Chimer. When the king of the Dwemer pushed his sword against Wulfarth's throat so he couldn't use the Voice, Dro'Zira leapt from beneath him and toppled the king, who fled into the Heart Chamber.

Behind Wulfharth came his legion of Tongues, powerful mages who used their Voice (I probably should have mentioned earlier and you might have realised by now but sound is sacred and magical in Mundus; The Towers themselves, reflections of the universe, are all related to music as well, with the Heart in Red Mountain being a drum, the Throat of the World in Skyrim being the Voice, the Tower in Cyrodiil, White-Gold, representing a harp and so on) to shout words of power in the dragon tongue, including the greatest Tongue of all, Jurgen Windcaller, who could topple walls with his words, while the Chimer's wizards were only matched by those of the Altmer while their warriors were, if I recall correctly, unmatched at the time, as the Redguards only came to Tamriel centuries later (unless I have my eras wrong). This great army showed up at Red Mountain and began to lay waste to the Dwemer there, while King Nerevar and his close advisors, Dagoth, Almalexia, Vivec and Sotha-Sil, marched into the Heart Chamber and confronted Dumac Dwarf-King, leader of the Dwemer, and his tonal architect (like a science-musician, sort of like how Redguards had sword-singers and Nords had shouty-warriors) Kagrenac. Nerevar fought Dumac and in the end Nerevar killed his former friend but was mortally wounded. Before Kagrenac could be dealt with he took his tools, a glove, a hammer and a knife, used them to tune the Heart, and at once every Dwarf on Nirn at the time (exactly one was in Oblivion, and was unaffected) disappeared, becoming the skin of a giant brass construct named Numidium or Anumidium and intended to be the god of the Dwemer and break reality using the Heart as its power source. Nerevar's dying wish to his four friends was that the tools of Kagrenac be taken away and kept somewhere safe, never used, so that the ruin that met the Dwemer could never meet the Chimer. He made his four friends swear an oath on Azura's name that they would see to this wish. Outside, the Nords' Voice apparently ceased to work. Probably had something to do with the immense (but inaudible) sound Kagrenac had made, but Jurgen blamed it on the overuse and misuse of the Voice to win battles, and from then on forbade the Nords from using the Voice except for worship and in times of absolute need, and since he was the greatest Tongue none could question his decision.

When Nerevar was seen to and passed away, Sotha, Vivec and Almalexia returned to find that Dagoth, who had been left to guard the tools, seemed mad and would not relinquish them. When they tried to cease the tools of Kagrenac by force he used them on the Heart. They managed to overpower and kill him there, taking the tools, but he did not truly die as he had used the Heart to make himself immortal, sensing his friends' treachery. Soon afterward, they came back to break their oath, and used the Tools and the Heart to become gods. For their treachery Azura cursed their followers, and the Chimer became the Dunmer.

So at this point we've got the Dwemer who traveled around in airships, made kinetic hammers that could fly across continents, made steam-powered centurion robots in magical armour and turned themselves into a giant Gurren Lagann sort of mech with the intention of breaking through reality itself and an android superweapon that can eat entire countries and has a lightsaber (forgot to mention that part about Pelinal, too lazy to go back and edit it; One of his hands had a "killing light" with which he eviscerated elves) and unfolds from a magical stone made by star-people, and we haven't even gotten to the Redguards blowing up Yokuda, the Marukhati Selective breaking Akatosh into eight different gods and ruining the flow of time, leading to the existence of time travel among other things, the various space programmes the races of Tamriel had or the technological advanced period of the late first era under the Reman Dynasty yet.