Tinuviel the name that Beren gave to Luthien, it is the poetic word for nightingale meaning 'Daughter of Twilight'.
Tinuviel daughter of Thingol and Melian of Doriath, the fairest Elf that ever lived. Some Elves believed that Arwen was the reincarnation of Tinuviel/Luthien. Tinuviel met Beren and fell in love with him as he with her, she wanted to marry him, but her father would only allow them to if Beren got the Silmaril from Melkor's hand. The King did this presuming that Beren would die, however this was not so, and with the help of Tinuviel and the wolf Huan he got the Silmaril. However Beren was then slain by Carcharoth a wolf of Angband, and Luthien passed away with grief.
Their spirits were taken to the Halls of Mandos where Lúthien sang a song that had great beauty and sorrow that moved even Mandos' heart. Because of this she was sent back to Middle-eath with Beren where she would become a mortal, they dwelt for a time in happiness on the green island of Tol Galen in the River Adurant.

In Fellowship of the ring, the book this is the song Aragorn tells the Hobbits about Luthien and Beren:
The leaves were long, the grass was green,
The hemlock-umbels tall and fair,
And in the glade a light was seen
Of stars in shadow shimmering.
Tinuviel was dancing there
To music of a pipe unseen,
And light of stars was in her hair,
And in her raiment glimmering.

There Beren came from mountains cold,
And lost he wandered under leaves,
And where the Elven-river rolled
He walked alone and sorrowing.
He peered between the hemlock-leaves
And saw in wonder flowers of gold
Upon her mantle and her sleeves,
And her hair like shadow following.

Enchantment healed his weary feet
That over hills were doomed to roam;
And forth he hastened, strong and fleet,
And grasped at moonbeams glistening.
Through woven woods in Elvenhome
She lightly fled on dancing feet,
And left him lonely still to roam
In the silent forest listening.

He heard there oft the flying sound
Of feet as light as linden-leaves,
Or music welling underground,
In hidden hollows quivering.
Now withered lay the hemlocks-sheaves,
And one by one with sighing sound
Whispering fell the beechen leaves
In the wintry woodland wavering.

He sought her ever, wandering far
Where leaves of years were thickly strewn,
By light of moon and ray of star
In frosty heavens shivering.
Her mantle glinted in the moon,
As on a hill-top high and far
She danced, and at her feet was strewn
A mist of silver quivering.

When winter passed, she came again,
And her song released the sudden spring,
Like rising lark, and falling rain,
And melting water bubbling.
He saw the elven-flowers spring,
About her feet, and healed again
He longed by her to dance and sing
Upon the grass untroubling.

Again she fled, but swift he came.
Tinuviel! Tinuviel!
He called her by her elvish name;
And there she halted listening.
One moment stood she, and a spell
His voice laid on her: Beren came,
And doom fell on Tinuviel
That in his arms lay glistening.

As Beren looked into her eyes
Within the shadows of her hair,
The trembling starlight of the skies
He saw there mirrored shimmering.
Tinuviel the elven-fair,
Immortal maiden elven-wise,
About him cast her shadowy hair
And arms like silver glimmering.

Long was the way that fate them bore,
O’er stony mountains cold and grey,
Through halls of iron and darkling door,
And woods of nightshade morrowless.
The Sundering Seas between them lay,
And yet at last they met once more,
And long ago they passed away
In the forest singing sorrowless.

I am not sure who did this painting/picture but i think it is great so i wanted it to be shown here!

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