Nienor, Tolkien's most tragic character, was born in Dor-Lómin during First Age, in the year of Nirnaeth Arnoediad. Her parents were Húrin Thalion, Lord of the House of Hador, Third House of the Edain, and Morwen Eledhwen of the House of Bëor, First House of the Edain. She had a sister, Lalaith who died before Nienor was born, and a brother, Túrin, who was sent to Doriath before her birth. Her name means Mourning.
She spent her childhood in Dor-Lómin, and when she was 20, as the road had become safer due to the Mormegil's exploits in Nargothrond, she went with her mother to Doriath with the hope to find her brother Túrin. But he had left Doriath a long time before and, under the name of Mormegil, defended Nargothrond.
After the fall of Nargothrond under the assaults of Glaurung, was heard the rumor that the Mormegil was Túrin. Morwen wished to be sure about it and see if he was still alive. Therefore, escorted by Mablung and a few elves from Doriath, she went to Nargothrond, not knowing Nienor accompanied them.
But Glaurung felt their presence and, diving into the Narog, he caused a cloud of foul vapour. The horses panicked, Mablung and his troop were lost in the fog. Morwen disappeared and Nienor was thrown to the ground but was not wounded. She reached Amon Ethir where she stared straight into the eyes of Glaurung. Using his power, he laid a spell of utter darkness and forgetfulness upon her. "She could remember nothing that had ever befallen her, nor her own name, nor the name of any other thing; and for many days she could neither hear, nor see, nor stir by her own will. Then Glaurung left her standing alone upon Amon Ethir, and went back to Nargothrond."
Thus Mablung and his companions found her. Returning to Doriath they were attack by Orcs at night, and Nienor, terrified, fled into the forest, tearing off all her clothes. Surprised by a storm, and in torror she cast herself upon the mound of Haudh-en-Elleth, where was buried Finduilas of Nargothrond.
That's where Turambar and the men of Brethil found her. But she could not remember her name, so Turambar named her Níniel, Tear-Maiden.
Getting back to Ephel Brandir on the next day, they came to Dimrost, where the Celebros fall towards Teiglin. A great shuddering came upon Níniel, wherefore afterwards the river was called Nen Girith, the Shuddering Water. She was sick of a fever and Brandir healed her and fell in love with her. But her heart was given to Turambar.
After three years had passed, Turambar married Níniel and in spring, Níniel conceived. At the same time rumors came at Ephel Brandir regarding the arrival of Glaurung.
Turambar made a daring plan to kill the dragon. He killed Glaurung in Cabed-en-Aras, but his hand was spat with the burning blood of the dragon and their eyes met: Túrin fell into a dark swoon.
Níniel, could wait no more. She went to search Turambar, followed by Brandir. Seeing him laying on the ground, she thought him dead. She tended his wounds and cried.
Glaurung stirred then for the last time ere he died and told her all the truth:
"Hail Nienor, daughter of Húrin. We meet again ere the end. I give thee joy that thou hast found thy brother at last. And now thou shalt know him: a stabber in the dark, treacherous to foes, faithless to friends, and a curse unto his kin, Túrin son of Húrin! But the worst of all his deeds thou shalt feel in thyself.
Then Glaurung died, and the veil of his malice was taken from her, and she remembered all the days of her life. Looking down upon Túrin she cried:
- Farewell, O twice beloved! A Túrin Turambar turun ambartanen: master of doom by doom mastered! O happy to be dead!
Then Brandir who had heard all, standing stricken upon the edge of ruin, hastened towards her; but she ran from him distraught with horror and anguish, and coming to the brink of Cabed-en-Aras she cast herself over, and was lost in the wild water.
Then Brandir came and looked down, and turned away in horror; and though he no longer desired life, he could not seek death in the roaring water. And thereafter no man looked again upon Cabed-en-Aras, nor would any beast or bird come there, nor any tree grow; and it was named Cabed Naeramarth, the Leap of the Dreadful Doom."
Sil, Chapter XXI - Túrin Turambar