Everlasting Darkness

Also given in the form 'Everlasting Dark', this was the fate that Fëanor called on himself and his Sons should they fail to fulfil their Oath to recover the Silmarils from Morgoth. Long afterward, when Fëanor himself and five of his seven Sons had been slain, the surviving two debated whether to continue their attempts to reclaim their father's Jewels. Maglor recalled the words of the Oath and its invocation of the Everlasting Dark, but his brother Maedhros claimed that, for all their desperate actions in pursuit of their Oath, the Darkness would take them whether they fulfilled the Oath or not.

It is far from clear what Fëanor meant by the 'Everlasting Darkness'. One obvious association would be with the Ancient Darkness, the Outer Void that surrounded the world, and perhaps Fëanor intended that those taking the Oath should be cast out of the world altogether if they failed. If so, that was not his fate: after his death, Fëanor's spirit returned to Mandos, but he was not permitted to take a new body, as would be normal for slain Elves. Indeed, perhaps this endless sojourn in the Halls of Waiting was what Fëanor had meant by the 'Everlasting Darkness'. If so, he succeeded in calling it down upon himself, and (if Maedhros was correct) upon his Sons as well.