So that is what the new series is about: the PT version of LOTR, the rise and fall of Annatar?
I know very little about the extended world of Middle Earth beyond the two obvious time periods that most know. I hope it will be interesting and not just GoT lite.
A little history for you on this.
Before Sauron was the big bad of Middle Earth, he was a lieutenant in the army of Morgoth. The War of Wrath broke out - the largest and longest in the history of Middle Earth. Morgoth's army squared off against the Army of Valinor, which consisted of predominantly Elves, but also men, dwarves and eagles. Morgoth commanded balrogs, dragons, orcs and more. The Army of Valinor suffered incredible losses, but did entirely wipe out Morgoth's army, save for Sauron, one balrog and a handful of orcs that fled.
One of the commanders in the Army of Valinor was Earendil, who had recruited the Valar (gods) to help defeat and capture Morgoth. He had two sons, Elros and Elrond. The Valar allowed the two sons to choose what life they would lead - Elrond obviously chose the life of the Elves and Elros chose the life of men. As thanks for their help defeating Morgorth and as reparations for the massive destruction in the Middle Earth, the Valar created an island for Elros to settle, which would become Numenor.
Over the years, the Numenoreans would grow to resent the choice Elros made, craving immortality and power. They cut themselves off from the Elves and the Valar, except for a handful that included Elendil and his sons, Anarion and Isildur.
The King of Numenor captured Sauron and brought him to Numenor as his prisoner - which Sauron planned in order to corrupt the Numenoreans. Over time, it worked, as Sauron used the Numenoreans desire for immortality against them, and convinced them to turn on the Valar and worship Morgoth, the dark Valar. Eventually, he would convince them to launch an attack on the Undying Lands.
Isildur learned of this and disguised himself and entered the King's palace and cut a fruit from the White Tree, that had originally been brought from the Undying Lands (Valinor) when Numenor was first settled by Elros. He was beaten within an inch of his life, but escaped with the fruit. He remained in a comatose state for months until a seed from the fruit began to grow and Isildur was suddenly healed. That very sapling would lead to the White Tree of Gondor, the tree of the King.
Elendil and his followed packed up and left Numenor, with the sapling of the White Tree, the palantirs and several other treasures, just as the rest of Numenor planned to launch their attack on Valinor. They left on ten ships, Elendil landed north in Middle Earth, Isildur and Anarion landed south. They founded the kingdoms that Aragorn would reunite thousands of years later.
Meanwhile, as soon as the Numenorean army touched down on the Undying Lands, they were swiftly destroyed by Eru (God) and Numenor was sunk into the sea, with only the faithful followers led by Elendil spared to continue the line. With Numenor destroyed, the greatest line of men was gone, and Sauron began to work to conquer Middle Earth. Disguised as Annatar, he forged the Rings of Power, controlled by the One Ring. The Elves could not be corrupted by them, but the nine kings of men who accepted them became the Nazgul.
Eventually this would all lead to a showdown with the War of the Last Alliance that ended the second age.