Morgoth FreedMorgoth Freed (4900 F.A.) - After his capture at the Battle of the Powers, Morgoth was given a 300 year sentence.

 

"But when the battle was ended and from the ruin of the North great clouds arose and hid the stars, the Valar drew Melkor back to Valinor, bound hand and foot, and blindfold; and he was brought to the Ring of Doom.  There he lay upon his face before the feet of Manwë and sued for pardon, but his prayer was denied, and he was cast into prison in the fastness of Mandos, whence none can escape, neither Vala, nor Elf, nor mortal Man.  Vast and strong are those halls, and they were built in the west of the land of Aman.  There was Melkor doomed to abide for three ages long, before his cause could be tried anew, or he should plead again for pardon." ~The Silmarillion, chap. 3 (Of the coming of the Elves)

 

And when his sentence was complete, he came once again before Manwë and pled for his freedom.

 

"Before the gates of Valmar Melkor abased himself at the feet of Manwë and sued for pardon, vowing that if he only be made the least of the free people of Valinor he would aid the Valar in all their works, and most of all in the healing of the many hurts he had done to the world.  And Nienna asked his prayer; but Mandos was silent.  Then Manwë granted him pardon." ~The Silmarillion, chap. 6 (Of Fëanor)

 

It would not be long before he was up to his old ways.