This period, lasting nearly three and a half thousand years, sets the stage for much of what we know in The Hobbit and The Lord of the Rings. It’s also the primary setting of Amazon’s upcoming series. Although Morgoth was destroyed in the War of Wrath, dark forces lingered in secret waiting to carry on his quest against the Valar and Eru Ilúvatar. But for the most part, the Second Age begins as a period of relative peace and rebuilding for the beings on Arda; the Elves begin slowly emigrating westwards, whether establishing themselves on Middle-Earth’s shores or emigrating to the lands of the Valar in Aman, while the Maiar Eönwë uplifted the remnants of the Edain and established the isle of Númenor. It wasn’t to last though. Sauron, one of Morgoth’s chief lieutenants, begins hatching his schemes to corrupt the Elves and Men about halfway through the Second Age.
Disguised as the Maiar Annatar, Sauron ensorcells the Elves of Eregion to forge the ring of power as he forges the One Ring to dominate those who use them. The discovery of Sauron’s deception kicks off a war between Sauron and the Elves of Middle-Earth that culminates in the second Dark Lord being routed back into Mordor. But centuries after, the Númenorean King Ar-Pharazôn—head of a Kingdom that now stretched beyond Númenor itself, and one that now distrusted the Elves—marched an army into Mordor to capture Sauron, imprisoning him on Númenor…where Sauron promptly began corrupting the Númenoreans, turning them against the Valar and convincing Ar-Pharazôn to attempt to land on Aman and attack the Valar. However, it’s a move that goes very poorly: Eru Ilúvatar shows up and is so mad he not only smashes the Númenoreans and Sauron’s physical form, he sinks Númenor entirely, and cuts off Aman from the rest of Arda. How does he do this? Thousand and thousands and thousands of years after actually having made it in the first place, he makes Arda a spherical planet instead of simply flat.
The remaining Númenoreans flee to Middle-Earth and establish the kingdoms of Gondor and Arnor, and after a few hundred more years, Sauron reforges his physical form. You know what happens next, because it’s how Fellowship of the Ring opens: the Last Alliance of Elves and Men is forged to defeat Sauron, culminating with his body once again being destroyed on the footsteps of Mount Doom by Prince Isildur.