Melkor Forevermore.
Morgoth
From the Ballantine paperback Silmarillion, p. 88 ("Then Feanor rose, and lifting up his hand before Manwe he cursed Melkor, naming him Morgoth, the Black Foe of the World; and by that name only was he known to the Eldar ever after.").
Note: a character can be killed by this riddle even if he gives a correct answer. Some riddles result in death if you answer them incorrectly. The 'Morgoth' riddle is one of these. I've gathered together quite a bit of evidence on this phenomena, and it appears that even giving the correct answer will kill a character 1 out of 3 times or so, regardless of allegiance. Just tough luck. If you get this riddle or another of the 'death' ones, just give the correct answer and pray.
Tolkien Gateway: Morgoth