Morgoth

"Eiliniel the fair thou shalt most surely find, and there where she doth dwell and wait for thee together you shall ye ever be, and sunderedshall ye sigh no more."

- Morgoth to Gorlim on Eiliniel Morgoth also called Melko is the Black Enemy of the World, a primordial source of evil and a dark lord who is the main antagonist of Beren and Lúthien.

Beren and Lúthien (Silmarilion)
"So Lúthien, so Lúthien a liar like all Elves and Men! Yet welcome, welcome to my hall! I have a use for every thrall. What news of Thingol in his hole shy lurking like a timid vole? What folly fresh is in his mind who cannot keep his offspring blind from straying thus? or can devise no better counsel to his spies?"

- Morgoth to Lúthien Morgoth lef Angband to corrupt the newly-awakened Men and coerce him into worshipping him, leaving Sauron to lord in his fortresses and wage war upon the Elves.

The dark lord Morgoth came to desire to learn of the whereabouts of King Barahir and his Men. To this end, Sauron had his spies search for him until they captured Gorlim. The Servant of Morgoth compelled the Man into revealing the location of the "rebel" by promising him to release his wife Eiliniel. Once he learned of it, Sauron had the traitor cruelly slain before sending the host of Morgoth to hunt down Barahir and his folk.

After Sauron's defeat in Tal-In Gaurhoth, Morgoth had learned that Beren and Lúthien had escaped and were hearding towards Angband. Knowing that Huan the Hound of Valinor might aid them, he bred Carcharoth the werewolf to be a match for the former. The Elf-maiden put his dark servants under a sleep-spell, before confronting the dark lord of Angband himself. After he attempted to ravish her, she put him into a deep slumber, allowing Beren to steal one of the Silmarils before Morgoth and his court awakened.

Beren and Lúthien (2017)
"Who art thou that flittest about my halls like a bat? How camest thou in, for of a surety thou dost not belong here?"

- Morgoth/Melko to Lúthien Melko's servants brought Beren before him when he was captured while seeking food in an encampment of Orcs. The Gnome convinced the dark lord of his unwavering loyalty and the Ainu of Evil spared him. However, Melko accepted him as a thrall and had him sent to serve under Tevildo the Prince of Cats.

Beren and Lúthien were able to pass Karkaras the wolfhound and enter the fortress. The Elf-maiden brought all of the King 's servants under a spell of sleep and eventually even Melko fell and the crown with the Silmarils fell also from his head, allowing Beren to take off one of the precious stones for his quest.

Earlier versions
In Beren and Lúthien of 2017, Melko/Morgoth is given the role of interrogating Gorlim, though in earlier versions and the one in the published Silmarilion it is Sauron who does this.

Appendices
"Melko - The great evil Vala, Morgoth (later form Melkor)"