Adar soon became disillusioned with the Dark Lord and disgusted by the selfish sacrificing of his "children" for dabblings in the Unseen World. Having had enough of slavery, Adar broke his shackles to Sauron and turned on him, going as far as to slay him.
After Morgoth's defeat, Adar at first served Sauron in the latter's mission to discover a way to subdue other forms of life. However, disillusioned by Sauron's cruelty to the Orcs, Adar turned on Morgoth's successor and struck him down, believing for centuries afterward that he had slain Sauron.
It revealed Adar no longer serves Sauron, as we initially thought the mutilated elf might. Throughout The Rings of Power, we've slowly uncovered Adar's true identity. The show has offered us insight into the one-time elf, his origins, and why the otherwise hateful orcs adore Adar as they do.
Initially an adherent of his second master's purposes to heal Middle-earth, he ultimately became disillusioned with the Dark Lord, noting the selfish sacrificing of his "children" to create a power over flesh. Adar eventually rebelled against Sauron, attaining the belief that he split him open.
Because of these future events in Middle-earth's plot concerning Sauron, it's far more likely that Adar never attacked Sauron, but he's lying to Galadriel about doing so to keep his current location and identity secret.
Because of his age, Adar would have had a prominent place in Morgoth's army, but he was still subservient to Sauron. So, when Sauron took over, Adar became his servant until Sauron started to experiment on Orcs. Because they were his children, Adar rebelled, and to his knowledge, they killed Sauron.
Adar revealed his entire story to Galadriel, and what he told her will change her mission in the Second Age. Adar is an original Orc, and he served Morgoth and Sauron. His life has been one of darkness, but his attack on the Southlands is a desperate attempt to create a home for his offspring.
The orcs are all Adar's offspring, and he cares for them, just as he is the only figure orcs truly care for in all of Tolkien's Middle-earth. The use of the word “Father” when it comes to Adar thus feels more emotional than just a factual title.
While riding, Halbrand seized a spear from a corpse, and maneuvering through through the forest, he ended up riding towards where Adar was heading. Useing the spear to trip the horse, Halbrand dismounted Adar, who attempted to provoke him into killing him. Halbrand was only just prevented from doing so by Galadriel.
There have been consistent reports he is a fallen Elf, a quite dramatic departure from Tolkien's own lore given he moved away from the idea that Elves can fall to the Shadow.
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.
Maeglin's father, Eöl, forged it first as Anglachel, but its final resting place remained above the seas at the end of the First Age on Tol Morwen. Thus, to recap: Adar is not actually Galadriel's bro, that's a rouse. Adar is simply a title for the Unwilling Elf, Maeglin.
Sauron's deepest fear was that Aragorn would use the Ring against him. With its powers of domination, Aragorn could comfortably take over the entirety of Sauron's dark army. Though this would eventually corrupt Aragorn, it would allow him to destroy Sauron and take his place.
So, Halbrand would hold a grudge if he is Sauron and Adar had tried to kill him (and perhaps severely injured him), but now Sauron has taken the form of Halbrand, then Adar would not recognise him.
In this way, Tolkien draws a very clear line between good and bad. The Rings of Power's Adar is a different proposition entirely. On one hand, Adar is absolutely a villain, as his body count in the Southlands can attest. His overall goal, however, is actually quite reasonable.
As for whether Halbrand actually loved or romantically desired Galadriel is to be determined. Sauron is an expert manipulator and likely sees Galadriel and her power as an asset. In line with the old adage, "Keep your friends close; keep your enemies closer," he'd rather have her on his side than against him.
"So I think in his pitch to Galadriel, it cannot mean that he loves her or that there's any kind of romantic relationship. There should be no ambiguity around the fact that Sauron is evil — he's terrible, and he's using Galadriel to enhance his power."
When Galadriel calls Adar's kind — captured Elves tortured and twisted into becoming Morgoth's evil minions — “Moriondor,” which translates to “sons of the dark” in Elvish, Adar says he prefers the term “Uruk,” which is the word for “Orc” in the Black Speech of Mordor.
The Elven identity of Adar also appears to rule him out as one of the Nazgul (or Ringwraiths) who go on to follow Sauron in the Lord of the Rings saga, as these were Kings of Men. Some had wondered if Adar would be the Witch-King of Angmar, but his Elven ears make this less likely.
If Adar was an Elf captured by Morgoth, he must've been one of the earliest Elves to awaken in Middle-earth. As the Enemy's prisoner, Adar and his fellow captives would've been tortured, twisted and corrupted into the very first Orcs - the Moriondor.
Adar is, after all, the race's progenitor from way back. Feeling protective over his little bundles of hate, Adar apparently killed Sauron, thus making himself the chief purveyor of Middle-earth misery.
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."
Although it's possible Adar is a weakened version of Sauron who hasn't yet recovered his full power, Joseph Mawle's character is more likely a follower of the Dark Lord than the man himself. The Rings of Power's scene between Adar and Arondir actually leans more toward the villain being a fallen elf.
Finally Sauron came out to fight Elendil and Gil-galad face to face. When Elendil fell, his sword Narsil broke beneath him. Isildur took up the hilt-shard of Narsil and cut the One Ring from Sauron's hand, vanquishing Sauron.
