§ Of Túrin Turambar The Silmarillion

Grieving Beleg looked upon him; for Gwindor was now but a bent and fearful shadow of his former shape and mood, when in the Nirnaeth Arnoediad that lord of Nargothrond rode with rash courage to the very doors of Angband, and there was taken. For few of the Noldor whom Morgoth captured were put to death, because of their skill in forging and in mining for metals and gems; and Gwindor was not slain, but put to labour in the mines of the North. By secret tunnels known only to themselves the mining Elves might sometimes escape; and thus it came to pass that Beleg found him, spent and bewildered in the mazes of Taur-nu-Fuin.'