§ Of Tuor and the Fall of Gondolin The Silmarillion

Now on a time, when Eärendil was yet young, Maeglin was lost. For he, as has been told, loved mining and quarrying after metals above all other craft; and he was master and leader of the Elves who worked in the mountains distant from the city, seeking after metals for their smithying of things both of peace and war. But often Maeglin went with few of his folk beyond the leaguer of the hills, and the King knew not that his bidding was defied; and thus it came to pass, as fate willed, that Maeglin was taken prisoner by Orcs, and brought to Angband, Maeglin was no weakling or craven, but the torment wherewith he was threatened cowed his spirit, and he purchased his life and freedom by revealing to Morgoth the very place of Gondolin and the ways whereby it might be found and assailed. Great indeed was the Joy of Morgoth, and to Maeglin he promised the lordship of Gondolin as his vassal, and the possession of Idril Celebrindal, when the city should be taken; and indeed desire for Idril and hatred for Tuor led Maeglin the easier to his treachery, most infamous in all the histories of the Elder Days. But Morgoth sent him back to Gondolin, lest any should suspect the betrayal, and so that Maeglin should aid the assault from within, when the hour came; and he abode in the halls of the King with smiling face and evil in his heart, while the darkness gathered ever deeper upon Idril.