Sauron

"Whom do you serve, Light or Mirk? Who is the maker of migthiest work? Who is the king of earthly kings, the greatest river of gold and rings? Who is the master of the wide earth?"

- Sauron/Thû Sauron also known as Thû the Necromancer or Gorthaur the Cruel is a lord of darkness, the Shadow of dread and the Enemy of the Valar and Free Peoples of Middle-earth who is the secondary antagonist of Beren and Lúthien.

Wars against the Elves
"Thou base, thou cringing worm! Stand up, and hear me! And now drink the cup that I have sweetly blent for thee! Thou fool: a phantom thou didst see that I, I Sauron, made to snare thy lovesick wits. Naught else was there. Cold'tis with Sauron's wraiths to wed! Thy Eiliniel! She is long since dead, dead, food of worms less low than thou. And yet thy boon I grant thee now: to Eiliniel thou soon shalt fo, and lie in her bed, no more to know of war - or manhood. Have thy pay!"

- Sauron to Gorlim, shortly before having the latter slain While Morgoth left to corrupt the newly-awakened Men and coerce them to worship him, Sauron took Angband as his fortress and waged war upon the Elves, until he forced King Finrod Feleagund to leave the Tower of Tol Sirion. After this Sauron, brief Lord of Angband, left his dark fortress and took the tower as his domain, settling there with his Werewolves and falling deeper into sorcery.

At the dark lord of Angband's command, the Servant of Morgoth began to male plans for bringing ruin upon the Men of King Barahir, who still dared to oppose his master. Sauron the Sorcerer used his dark spells to conceive a ghost of Eiliniel, the wife of Gorlim, which he used to lure the Man compell him into revealing the location of Barahir. Once he learned of it, he revealed to the traitor that Eiliniel was dead and then had him slain before sending his Orcish servants to destroy the hiding place.

The guards of Sauron later captured the company of Finrod Feleagund, which was disguised to look like orcs. Realizing who they were, the dark lord of Tol-In Gaurhoth had the Elves and Beren thrown into the dungeons to be devoured by his hungry werewolves. When Lúthien and Huan the Hound of Valinor came to rescue Beren, Gorthaur sent his wolves to take the maiden, for in his lust he desired to possess her or if not he would give her over to Morgoth. Draugluin the Sire of Werewolves returned to him empty-handed, and revealed that Huan was with her, before dying at his lord's feet. Knowing far too well the fate of the latter, Sauron took the form of a great werewolf and attacked the intruders. He was defeated and he yielded the keys of Tol-In Gaurhoth to Lúthien before fleeing in the form of a great vampire and hiding either because he feared the wrath (and scorn) of Morgoth or the Valar.

Beren and Lúthien (2017)
"Death to light, to law, to love! Cursed be moon and sky above! May darkness everlasting old that waits outside in surges cold drown Manwë, Varda, and the sun! May all in hatred be begun and all in evil ended be, in the moaning of the endless Sea!"

- Sauron/Thû The guards of Thû the Necromancer later captured the company of Finrod Feleagund, which was disguised to look like orcs. Realizing who they were, the dark lord of Tol-In Gaurhoth had the Elves and Beren thrown into the dungeons to be devoured by his hungry werewolves. When Lúthien and Huan the Hound of Valinor came to rescue Beren, he sent his wolves to take the maiden, for in his lust he desired to possess her or if not he would give her over to Morgoth. Draugluin the Sire of Werewolves returned to him empty-handed, and revealed that Huan was with her, before dying at his lord's feet. Knowing far too well the fate of the latter, Thû took the form of a great werewolf and attacked the intruders. He was defeated and he yielded the keys of Tol-In Gaurhoth to Lúthien before fleeing in the form of a great vampire and hiding either because he feared the wrath (and scorn) of Morgoth or of the Valar.

Aftermath
"Men called him Thû, and as a god in days after bewildered bowed to him, and made his ghastly temples in the shade"

- On Sauron/Thû's eventual worship by Men Thû would eventually return from hiding. He would later enthrall Men and coerce them into worshipping him. These cultists would later make temples to honor their "god", for he was no more the Servant of Morgoth but a dark lord in his own right.

Earlier versions of the tale
In the original Lay of Leithian, the character of Tevildo the Prince of Cats was the forerunner to Sauron, acting as the secondary antagonist besides Melko (Morgoth), keeping Beren his captive and being later defeated by Huan and forced to flee.

In later versions, the character of Thû appeared and he ultimately became the dark lord Sauron. In the latest version from 2017 edited and published by J.R.R. Tolkien's son Christopher Tolkien, Tevildo is introduced as a separate character and Thû is the name by which the main heroes address Sauron. His role however as the one to interrogate Gorlim is given instead to Morgoth himself.

Appendices
" Thû - The Necromancer, greatest of the servants of Morgoth, dwelling in the Elvish watchtower Tol Sirion, later name Sauron "