Originally a Maia of Aulë's people, Sauron was early corrupted by Melkor and became his most trusted lieutenant. In the wars of Beleriand, Sauron, master of phantoms and shadows, was the most feared of Morgoth's servants, but after the War of Wrath and the expulsion of the first Dark Lord, Sauron rose to become the Dark Lord, greatest enemy of Elves and Men in the Second and Third Ages.
Sauron is important in Aragorn's history because Sauron was partly responsible for the downfall of Númenor, where Aragorn's Dúnedain ancestors fled from. Sauron was defeated by Aragorn's line three times. The Dúnedain owned the seven palantír stones that were brought to Middle-earth, and so Aragorn was able to wrest control of the palantír from Sauron.
Sauron is one of most powerful Maiar whose power only grew stronger with the forging of the One Ring. In the First Age he was know as Gorthaur, Warden of Angband, and first in command after Morgoth.
During the wars of Beleriand,, Sauron conquers wins the stronghold of Minas Tirith; which was a great tower built between two mountain ranges and an elven outpost against Angband. From there he commanded orcs, wargs and ancient werewolfes. But he was defeated by Huan and Lúthien, who had come to save Beren from captivity. Sauron was able to change into many different formes, for example a Master of Werewolfes in battle with Huan, and after, a great vampire while he was escaping back to Angband.
With the fall of Morgoth, Sauron hid in the deeps of Angband so he escaped the Valar and elves in the War of Wrath. Many years later he showed himself again, taking on the mantle of the heir of Melkor.
He began revealing himself once more, and by SA 1000 he gathered his power and established himself in the land of Mordor in eastern Middle-earth and began building the dreaded Dark Tower of Barad-dûr near Mount Doom. Around SA 1500, Sauron put on a fair visage and called himself Annatar (or Antheron), the "Lord of Gifts", thus befriending the Elvish smiths of Eregion. Sauron was impersonating a minister of the Valar, a craftsman of great power and knowledge. He taught the elf-smiths to craft magic rings of power, and Celebrimbor the leader of Eregion crafted three elven rings called Vilya, Narya and Nenya which he gave to three great elf lords: Galadriel, Elrond and Gil-Galad. In secret Sauron returned to Mordor where he, in the fires of Mount Doom, forged a Master Ring, the One Ring to Rule them All. He forged it from the fire and he crowned it with the darkness. He put it on with words: '"sh nazg durbatulûk, ash nazg gimbatul. Ash nazg thrakatulûk agh burzum-ishi krimpatul." in language of man: "One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them." The Elves heard Sauron and they saw his treachery so they hid the three elven rings far from his reach. In Eregion when Celebrimbor forged the three Rings, Sauron made many other, lower rings. They were given to the other races, Nine to Men and Seven to Dwarves, and Sauron mastered them. As the ancient lyrics say:
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,
In the Land of Mordor where the Shadows lie,
One ring to rule them all, one ring to find them,
One ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.
Three were hidden, seven lost and nine, nine for men were given to great human kings and sorcerers but under Sauron control they became know as the Nazgûl, the most deadly servants of the Enemy. In the One ring, Sauron installed a great part of his power so the destruction of the ring would severely weaken its master, but ring was giving him great power while he wielded it. With it, Sauron showed his true colors, gathered his armies and began a great war for Middle-Earth.
Sauron himself makes a small appearance in Vol I. Book 9 as the red eye, when the player escorts Sara Oakheart through Barad Gûlaran, where she later reveals herself as Amarthiel and asks him where the ring Narchuil is. He also appears as the red eye in the instance Sammath Gûl, as the players approach Gorothúl, the third boss.
note: in the session play quest 'Daughter of Strife' from Volume I: Book XV, where the player takes on the character of Narmeleth, Sauron is also encountered in his disguise of Antheron, at the time he taught the elves of Eregion the secrets of the rings.