Treachery of Men
The Treachery of Men refers to the betrayal by the House of Ulfang at the Nirnaeth Arnoediad. However the House of Ulfang was already in the service of Morgoth for many years prior, masquerading as being loyal to the Sons of Fëanor. When Morgoth learnt of the battle-plan of the Union of Maedhros for the Nirnaeth, he ordered those traitors to delay the advance of the Eastern Host lead by Maedhros as they pushed west as part of the plan to flank Morgoth's armies. The mass defection fatally slowed Maedhros' host, stopping them from relieving Fingon's forces in the west and allowing the vast army Morgoth's enemies had assembled to be destroyed piecemeal. Though the House of Bór hunted down and slew many of Ulfang's treacherous clan, the damage was done: The Treachery of Men effectively lost the Ñoldor and their allies the battle (to what degree depending on which version: see Nirnaeth Arnoediad) and the deed was the cause for the estrangement of the Easterlings and other Mannish groups (except the Edain) from the Elves.