The calendar of Arda used to employ Valian Years, each of which is approximately equal to ten Solar Years. As such, the First Age begins around 48,000 years following the Valar's arrival. The world undergoes a significant metamorphosis over the course of these 48 millennia, including mountain formation and forest development.
The Elves are born only after Arda becomes ecologically self-sufficient, allowing them to engage with the environment on both physical and spiritual levels. The firstborn Children of Ilúvatar are soon followed by Men, Dwarves, Hobbits, Orcs, Trolls, and other sentient races. Despite the tumultuous events of the First Age, The Rings of Power takes place entirely during the Second Age.
10 The Construction Of The Grey Havens
Originally known as Mithlond, the Grey Havens were built in the first year of the Second Age. Elvish survivors from Beleriand were largely responsible for its foundation, although other clans also lent their support. The Númenóreans frequently visited this thriving port city, and even sent military aid to Middle-earth.
Círdan the Shipwright, a former wielder of Narya, governed the Grey Havens for several hundred years, and willingly chose to remain behind when Gandalf, Galadriel, and Elrond made their way back to Aman. Every ship that travels to Valinor from Middle-earth starts its journey at the Grey Havens.
9 The Rise Of Númenor In The West
The marvelous Kingdom of Númenor was constructed in SA 32. The Valar used their magic to extract a landmass from the Belegaer, or the Sundering Sea, and graciously offered it to the newly minted House of Elros Tar-Minyatur.
Shaped like a five-pointed star, the island of Númenor was rapidly populated by the Dúnedain and their descendants, who went on to create the greatest realm of Men on Middle-earth. Headquartered at Armenelos, the line of Númenorean Kings lasted for twenty-five generations before the whole island was wiped off the map.
8 The Second Coming Of Sauron
Sauron was born in the Timeless Halls of Eru Ilúvatar and existed long before the world came into being. He spent his first few millennia working for Aulë, but Melkor managed to tempt him away from the righteous path. Sauron served this fallen Vala throughout the First Age, at least until Morgoth was defeated, captured, chained, and banished from Eä.
Sauron concealed his presence for the first five hundred years of the Second Age, but eventually crept out of the darkness in order to complete his former master's mission. Sauron's resurgence spreads discord in Middle-earth, especially when he executes his magic jewelry scam.
7 The Forging Of The Rings Of Power
The epigraph to The Lord of the Rings reads: "Three Rings for the Elven-kings under the sky, Seven for the Dwarf-lords in their halls of stone, Nine for Mortal Men doomed to die, One for the Dark Lord on his dark throne." These are the only twenty rings with canonical value, forged in the fifteenth century of the Second Age.
Many others supposedly exist, but their histories are lost to time. The Dwarves were somewhat immune to Sauron's malevolent aura, and the Elves quickly recognized his evil designs, but the Men weren't so lucky. The Nine chosen humans gradually metamorphosed into living wraiths, their souls eternally enslaved to Sauron.
6 The Birth Of Rivendell
Imladris, or Rivendell, is correctly called the "Last Homely House East of the Sea." Like Gondolin in the First Age, this tranquil township was designed to be a refuge for the Elves. Elrond founded Rivendell in SA 1697, at the height of the War of the Elves and Sauron, which had completely destroyed the Ñoldorin realm of Eregion. Sauron lost the war, and thus never discovered Rivendell's location.
Isildur's youngest son Valandil was later raised under Elrond's tutelage, as were many of his descendants, including Aragorn Elessar. Bilbo Baggins described Rivendell as "a pefect house, whether you like sleep, or storytelling or singing, or just sitting and thinking best, or a pleasant mixture of them all."
5 The Emergence Of The Nazgûl
According to Gandalf, the Nine Rings converted their wearers into "Ringwraiths, shadows under his great Shadow, [Sauron's] most terrible servants." The Nazgûl first materialize in SA 2251, 558 years after receiving their respective rings.
Nobody knows the events that transpired during this six-century time interval, but it can be safely assumed that their transformations into Ringwraiths weren't pleasant. The One Ring controls the Nine Rings and, by extension, the Nazgûl. While these hapless creatures truly don't have a choice in the matter, it doesn't mean their horrifying crimes can be forgotten or forgiven.
4 The Tyranny Of Ar-Pharazôn
Ar-Pharazôn is a major character in The Rings of Power, although he's yet to showcase the depth of his depravity. In The Akallabêth, Tolkien blames the Downfall of Númenor on Ar-Pharazôn the Golden. The last King of Númenor allows himself to be swayed by Sauron's honeyed words, and pompously decides to declare war on Valinor in SA 3319.
In a desperate bid to evade his own mortality, Ar-Pharazôn foolishly flouts the Ban of the Valar, leaving Ilúvatar no choice but to intervene. The "King and the mortal warriors that had set foot upon the land of Aman were buried under falling hills." Ar-Pharazôn's condemned soul is permanently trapped inside the Caves of the Forgotten, and he will not be freed until the Dagor Dagorath.
3 The Changing Of The World
Ar-Pharazôn invoked The One's divine wrath, who retaliated with a mega-tsunami and a series of devastating earthquakes. Although many Númenoreans were innocent, they were forced to pay the price for their King's misdeeds. Ilúvatar "changed the fashion of the world," by carving an enormous abyss in the Sundering Sea.
The island's lore, art, traditions, towers, gardens, and children, "its laughter and its mirth and its music," were instantly and indifferently swallowed up by the ocean. More importantly, Ilúvatar relocated Eressëa and Valinor beyond the geographical purview of Men, preventing them from accessing the Undying Lands ever again. The world of Arda, originally a flat circle, became a globe overnight.
2 The Desecration Of The White Tree
Armenelos was constructed by the Edain, who were allegedly assisted by a group of unnamed Maia. This sprawling capital city of Númenor, ruled by Queen Regent Míriel, appears in The Rings of Power. The iconic White Tree is shown in a few scenes, dispersing its fragrant blossoms all over town.
In the books, the Elves of Tol Eressëa offered this tree to the Edain of Westernesse, commemorating an alliance between the two races. In SA 3262, Sauron convinced Ar-Pharazôn to use the White Tree as kindling for the sacrificial fire at the Temple of Morgoth. However, Isildur secretly stashed a single fruit of the White Tree before it was utterly destroyed. The White Tree of Gondor is descended from its Númenorean counterpart.
1 The War Of The Last Alliance
The War of the Last Alliance is briefly covered in The Fellowship of the Ring (2001), which describes how Sauron killed both Elendil and Gil-Galad. During the desperate attempt to avenge his father's death, Isildur managed to cut the One Ring off Sauron's hand, leading to the Dark Lord's destruction.
This earth-shattering conflict actually raged from SA 3429 to 3441, an extremely turbulent period for Middle-earth. A massive amalgamated army composed of Elves, Men, and Dwarves fought bravely against the forces of Mordor. The seven-year-long Siege of Barad-dûr was finally broken after Sauron entered the battlefield — and the rest is history.