Elrond and the Last Sons of Fëanor (FR 2.iii.281)

As the Fellowship is about to depart Rivendell, Elrond offers them these final words:

'... no oath or bond is laid on you to go further than you will. For you do not yet know the strength of your hearts, and you cannot foresee what each may meet upon the road.' 

'Faithless is he that says farewell when the road darkens,' said Gimli.  

'Maybe,' said Elrond, 'but let him not vow to walk in the dark, who has not seen the nightfall.'  

'Yet sworn word may strengthen quaking heart,' said Gimli.  

'Or break it,' said Elrond. 

(FR 2.iii.281)

Few among the living in Middle-earth would have known this truth as well as Elrond. For he had been raised by Maedhros and Maglor (Silmarillion 246-247; HoME IV 150, 153, 162, 309), the last surviving sons of Fëanor, who were driven to heartbreak, murder, and their own destruction by the oath they had sworn to regain the Silmarils at any cost:

Then Eönwë as herald of the Elder King summoned the Elves of Beleriand to depart from Middle-earth. But Maedhros and Maglor would not hearken, and they prepared, though now with weariness and loathing, to attempt in despair the fulfilment of their oath; for they would have given battle for the Silmarils, were they withheld, even against the victorious host of Valinor, even though they stood alone against all the world. And they sent a message therefore to Eönwë, bidding him yield up now those jewels which of old Fëanor their father made and Morgoth stole from him. 

 But Eönwë answered that the right to the work of their father, which the sons of Fëanor formerly possessed, had now perished, because of their many and merciless deeds, being blinded by their oath, and most of all because of their slaying of Dior and the assault upon the Havens. The light of the Silmarils should go now into the West, whence it came in the beginning; and to Valinor must Maedhros and Maglor return, and there abide the judgement of the Valar, by whose decree alone would Eönwë yield the jewels from his charge. Then Maglor desired indeed to submit, for his heart was sorrowful, and he said: The oath says not that we may not bide our time, and it may be that in Valinor all shall be forgiven and forgot, and we shall come into our own in peace.  

But Maedhros answered that if they returned to Aman but the favour of the Valar were withheld from them, then their oath would still remain, but its fulfilment be beyond all hope; and he said: 'Who can tell to what dreadful doom we shall come, if we disobey the Powers in their own land, or purpose ever to bring war again into their holy realm?'  

Yet Maglor still held back, saying: 'If Manwë and Varda themselves deny the fulfilment of an oath to which we named them in witness, is it not made void?'  

And Maedhros answered: 'But how shall our voices reach to Ilúvatar beyond the Circles of the World? And by Ilúvatar we swore in our madness, and called the Everlasting Darkness upon us, if we kept not our word. Who shall release us?' 

'If none can release us,' said Maglor, 'then indeed the Everlasting Darkness shall be our lot, whether we keep our oath or break it; but less evil shall we do in the breaking.' 

Yet he yielded at last to the will of Maedhros, and they took counsel together how they should lay hands on the Silmarils. And they disguised themselves, and came in the night to the camp of Eönwë, and crept into the place where the Silmarils were guarded; and they slew the guards, and laid hands on the jewels. Then all the camp was raised against them, and they prepared to die, defending themselves until the last. But Eönwë would not permit the slaying of the sons of Fëanor; and departing unfought they fled far away. Each of them took to himself a Silmaril, for they said: 'Since one is lost to us, and but two remain, and we two alone of our brothers, so is it plain that fate would have us share the heirlooms of our father.'  

But the jewel burned the hand of Maedhros in pain unbearable; and he perceived that it was as Eönwë had said, and that his right thereto had become void, and that the oath was vain. And being in anguish and despair he cast himself into a gaping chasm filled with fire, and so ended; and the Silmaril that he bore was taken into the bosom of the Earth. 

And it is told of Maglor that he could not endure the pain with which the Silmaril tormented him; and he cast it at last into the Sea, and thereafter he wandered ever upon the shores, singing in pain and regret beside the waves. For Maglor was mighty among the singers of old, named only after Daeron of Doriath; but he came never back among the people of the Elves. And thus it came to pass that the Silmarils found their long homes: one in the airs of heaven, and one in the fires of the heart of the world, and one in the deep waters. 

(Silmarillion 253-54)

Elrond thus appears not to be merely trading proverbial barbs with Gimli, but alluding to personal knowledge of the destructive power oaths can wield.