The Oath of Fëanor, The Silmarillion, pages 97-98:
Then Fëanor swore a terrible oath. His seven sons leapt straightaway to his side and took the selfsame vow together, and red as blood shone their drawn swords in the glare of the torches. They swore an oath which none shall break, and none should take, by even the name of Ilúvatar, calling the Everlasting Dark upon them if they kept it not; and Manwë they named in witness, and Varda, and the hallowed mountain of Taniquetil, vowing to pursue with vengeance and hatred to the ends of the World Vala, Demon, Elf or Man as yet unborn, or any creature, great or small, good or evil, that time should bring forth unto the end of days, whoso should hold or take or keep a Silmaril from their possession. Thus spoke Maedhros and Maglor and Celegorm, Curufin and Caranthir, Amrod and Amras, princes of the Noldor; and many quailed to hear the dread words. For so sworn, good or evil, an oath may not be broken, and it shall pursue oathkeeper and oathbreaker to the world's end.

The Silmarillion, pages 103-104:
Tears unnumbered ye shall shed; and the Valar will fence Valinor against you, and shut you out, so that not even the echo of your lamentation shall pass over the moutains. On the House of Fëanor the wrath of the Valar lieth from the West unto the uttermost East, and upon all that will follow them it shall be laid also. Their Oath shall drive them, and yet betray them, and ever snatch away the very treasures that they have sworn to pursue. To evil end shall all things turn that they begin well; and by treason of kin unto kin and the fear of treason, shall this come to pass. The Disposessed shall they be forever.
Ye have spilled the blood of your kindred unrighteously and have stained the land of Aman. For blood ye shall render blood, and beyond Aman ye shall dwell in Death's shadow. For though Eru appointed to you to die not in Eä, and no sickness may assail you, yet slain ye may be, and slain ye shall be: by weapon and by torment and grief; and your houseless spirits shall come then to Mandos. There long shall ye abide and yearn for your bodies, and find little pity though all whom ye have slain should entreat for you. And those that endure in Middle-earth and come not to Mandos shall grow weary of the world as with a great burden, and shall wane, and become as shadows of regret before the younger race that cometh after. The Valar have spoken.
This is the Doom of the Noldor, and it haunts them still.

Tolkien worked on The Silmarillion from 1916 until his death in 1973, forever revising and rewriting it. It was eventually published in 1977 by his son Christopher Tolkien after some work to tie it all together. It starts with the Music of the Ainur, detailing the music that created Arda and the first rebellions of Melkor, the mightiest of the Valar, to the fateful words of Ilúvatar (The Silmarillion, page 21):
Therefore I say: Eä! Let these things Be! And I will send forth into the Void the Flame Imperishable, and it shall be at the heart of the World, and the World shall Be; and those of you that will may go down into it.
Also in the Music of the Ainur, is the first battles over Middle-Earth in which the shaping of Arda was finished. The end of The Silmarillion is also the end of the Third Age, with the entire history of the Rings of Power that was recounted in part in the Council of Elrond though not written in Bilbo's account (The Lord of the Rings, page 259): Then through all the years that followed he traced the Ring; but since that history is elsewhere recounted, even as Elrond himself set it down in his books of lore, it is not here recalled.
It is instead, in The Silmarillion.

Many beautiful things were achieved in the First and Second Ages of the Sun, not the least of which, Gondolin (shown on the left) survived long, though like much else eventually fell to Morgoth. Not only was much of beauty achieved, but much of evil, too, as was said in the Doom of the Noldor, which many believe to have been spoken by Mandos. Not the least of the evil deeds include the Kinslaying at Alqualondë, from which many of the later deeds were rooted, the slaughter of Dior, by the Sons of Fëanor and the breaking of the Ban of the Valar, by the Númenoreans in the Second Age.

Many brave deeds were accomplished and many cowardly deeds too. Of the brave, one of the best known includes the fight between Fingolfin, the High King of the Noldor and Morgoth:
Therefore Morgoth came, climbing slowly from his subterranean throne, and the rumor of his feet was like thunder underground. And he issued forth clad in black armour; and he stood before the King like a tower, iron-crowned, and his vast shield, sable unblazoned, cast a shadow over him like a stormcloud. But Fingolfin gleamed beneath it as a star; for his mail was overlaid with silver, and his blue shield was set with crystals; and he drew his sword Ringil, that glittered like ice.
Then Morgoth hurled aloft Grond, the Hammer of the Underworld, and swung it down like a bolt of thunder. But Fingolfin sprang aside, and Grond rent a mighty pit in the earth whence smoke and fire darted. Many times Morgoth essayed to smite him, and each time Fingolfin leaped away, as a lighting shoots from under a dark cloud; and he wounded Morgoth with seven wounds, and seven times Morgoth gave a cry of anguish, whereat the hosts of Angband fell upon their faces in dismay, and the cries echoed in the Northlands.
But at last the King grew weary, and Morgoth bore down his shield upon him. Thrice he was crushed to his knees, and thrice arose again and bore up his broken shield and stricken helm. But the earth was all rent and pitted about him, and he stumbled and fell backward before the feet of Morgoth; and Morgoth set his left foot upon his neck, and the weight of it was like a fallen hill. Yet with his last and desperate stroke Fingolfin hewed the foot with Ringil, and the blood gushed forth black and smoking and filled the pits of Grond.
Thus died Fingolfin, High King of the Noldor, most proud and valiant of the Elven-Kings of old. The Orcs made no boast of that duel at the gate; neither do the Elves sing of it, for their sorrow is too deep.
The Silmarillion, pages 184-185.

The cowardly deeds include the two kinslayings. The first was at Alqualondë when Fëanor stole the white ships of the Teleri and the second was in Doriath:
They came at unawares in the middle of winter, and fought with Dior in the Thousand Caves; and so befell the second slaying of Elf by Elf. There fell Celegorm by Dior's hand, and there fell Curufin, and dark Caranthir; but Dior was slain also, and Nimloth his wife, and the cruel servants of Celegorm seized his young sons and left them to starve in the forest. Of this Maedhros indeed repented, and sought long for them in the woods of Doriath; but his search was unavailing, and of the fate of Elured and Elurin no tale tells.
The Silmarillion, page 286.

One of the most moving tales told in the Silmarillion is that of Beren and Lúthien. Given here is the brief version told in The Lord of the Rings pages 208-209, though it is also told as the Lay of Lethian and The Gest of Beren and Lúthien in The Lays of Beleriand:

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.
Tinúviel 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 quavering.
Now withered lay the hemlock-sheaves,
And one by one with sighing sound
Whispering fell the beachen 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,
Tinúviel! Tinúviel!
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 Tinúviel
That in his arms lay glistening.

As Beren looked into her eyes
Within the shadows of her hair,
The trembling starlight of her hair,
He saw there mirrored shimmering.
Tinúviel 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.

The Silmarillion is the history of Middle-Earth from the creation of Arda to the end of the Third Age. There are many tales that were recorded by the Eldar and then told to Men who recorded them that we would not otherwise know. These include the tales from the Ages of the Trees and those of the creation of Arda as well as all of the tales predating the Awakening of Men with the Rising of the Sun.