In the Second Age, Sauron frequently disguised in "a fair form", and took the false identity of Annatar, "Lord of gifts", among the Elves and in the kingdom of Númenor, which he intended & succeeded to destroy from within.
Following this betrayal, Sauron assumed the identity of "Halbrand", posing as a lone wanderer from the Southlands who had fled when Adar's forces had invaded the territory.
Tolkien depicted Annatar as a figure beautiful in appearance, with something not quite right about him. In Frodo's words, "a servant of the Enemy would look fairer and feel fouler.” Tolkien described Sauron's Annatar disguise as a "specious fair form” in Unfinished Tales.
Realising he could not defeat the Númenóreans with military strength, Sauron allowed himself to be taken as a hostage to Númenor by King Ar-Pharazôn in S.A. 3262. There, he quickly grew from captive to adviser and was known as Zigûr, the Wizard; he seduced the King and further corrupted the people.
Instead, Sauron did not redeem himself in any way. But the option seemed to have been in place for it to occur, which gave "The Rings of Power" a starting point to build on. The creation of the Halbrand identity seemingly served as a way for Sauron to start a new life in Middle-earth.
Halbrand refused to reveal his true identity to Adar
He served under Morgoth and claims that he killed Sauron. He does not seem to recognize Halbrand as Sauron, and yet he carries out what we can assume is his master's evil bidding by setting in motion the events that burn the Southlands and create Mordor.
Halbrand does not deny his heroism out of bashful humility, though; he knows that as Sauron, he's not the hero she wants him to be for the Southlands -- not when his plans involve scorching it into Mordor. No, Halbrand was never a hero, nor did he ever claim to be.
The Númenóreans brought him back as a prisoner. Through Sauron's cunning and persuasion he rose from prisoner to adviser for the King, convincing many officials to worship Morgoth, they quickly became corrupted to his side.
Because of this letter, it is quite clear that not only did Sauron carry the Ring while he was in Númenor, it was actually the Ring's power that allowed him to so quickly corrupt the kingdom.
Morgoth, the first to reign terror upon Middle-earth, was more powerful than Sauron and commanded an army of dragons and Balrogs. While both Morgoth and Sauron were cunning, Morgoth's rule and terror lasted longer and his corruption spread further, making him more formidable in the Lord of the Rings universe.
When Halbrand and Galadriel captured Adar, Adar seemed not to recognize Halbrand as Sauron. Adar also claimed to have killed Sauron after watching Sauron experiment on the orcs, whom Adar sees as his children. It's possible Adar was pretending not to recognize Sauron as part of a ruse.
Elrond discovered the scroll that revealed that Halbrand couldn't be King of the Southlanders, so it's likely that he realized that Halbrand was Sauron and that he escaped. Knowing that Sauron is on the loose, he could warn the dwarves of the growing evil and encourage them to forge their own rings.
Bloodlines are very important in Tolkien's stories, and it makes a lot of sense that Sauron would fear the great-grandson of the man who cut off his hand. Aragorn carried the blood of Numenor with him. His genetic connection to the heroes of the Second Age established him as a dangerous opponent of Sauron.
This revelation refutes any lingering theories that The Stranger (Daniel Weyman), the man who fell from the sky at the start of the season, is in fact Sauron, with the finale confirming he's actually one of the Istari, Tolkien's version of Wizards.
In the Second Age, Sauron frequently disguised in "a fair form", and took the false identity of Annatar, "Lord of gifts", among the Elves and in the kingdom of Númenor, which he intended & succeeded to destroy from within.
Eventually, Sauron surrendered to the Númenóreans and was their prisoner, but using his cunning he quickly became an adviser to the King. On Númenor, Sauron is known as Zigûr, meaning sorcerer, and he used his position to convert many Númenóreans to the darkness.
Although Sauron's body was destroyed in the Fall of Númenor, his spirit was able to bear the Ring back to Middle-earth and he wielded it in his renewed war against the Last Alliance of Elves and Men between SA 3429 and 3441.
Sauron did not fear as we think of the word. He was very cautious of the men of Numenor and the remaining Noldor in exile. No other force seemed to give him pause since the First Age. This was his undoing.
Sauron's forces were driven back from the Grey Havens, and retreating to the Gwathló found their way blocked by the second Númenórean army. Sauron's army was thus pinned between the two forces and mostly wiped out in the ensuing Battle of the Gwathló.
For The Rings of Power, Númenórian hatred of Elves built over time out of jealously for their immortality. In The Silmarillion, Tolkien outlines the downfall of Númenor as a slow burn that began with the Ban of the Valar, which entailed the first Númenórians agreeing to never sail west toward Valinor.
At some point prior to The Rings of Power, Sauron conjured a fresh physical form. He then seized the Southlands' crest from its last known king (who died 1000 years prior to the TV show's timeline) and assumed the identity "Halbrand."
Surprise! Halbrand is Sauron. The truth was revealed after Galadriel became suspicious of the supposed Southlander's sudden sway over Celebrimbor early in the season finale. One phrase, "a power over flesh", was imparted by Halbrand and led Galadriel to do some digging of her own.
In the final minutes, Sauron confesses that he's been manipulating Galadriel since they first met — using her as a way to get back to Middle-earth to fight back against his enemies in the Southlands and indirectly form the land of Mordor but also to get closer to Celebrimbor and the Elven smiths.
