The following is taken from Myths of the Norsemen: From the Eddas and Sagas by H. A. Guerber available from Project Gutenberg.

The Decline of the Gods

One of the distinctive features of Northern mythology is that the people always believed that their gods belonged to a finite race. The Æsir had had a beginning; therefore, it was reasoned, they must have an end; and as they were born from a mixture of the divine and giant elements, being thus imperfect, they bore within them the germ of death, and were, like men, doomed to suffer physical death in order to attain spiritual immortality.

The whole scheme of Northern mythology was therefore a drama, every step leading gradually to the climax or tragic end, when, with true poetic justice, punishment and reward were impartially meted out. In the foregoing chapters, the gradual rise and decline of the gods have been carefully traced. We have recounted how the Æsir tolerated the presence of evil, personated by Loki, in their midst; how they weakly followed his advice, allowed him to involve them in all manner of difficulties from which they could be extricated only at the price of part of their virtue or peace, and finally permitted him to gain such ascendency over them that he did not scruple to rob them of their dearest possession, purity, or innocence, as personified by Balder the good.

Too late the gods realised how evil was this spirit that had found a home among them, and too late they banished Loki to earth, where men, following the gods' example, listened to his teachings, and were corrupted by his sinister influence.

"Brothers slay brothers;
Sisters' children
Shed each other's blood.
Hard is the world;
Sensual sin grows huge.
There are sword-ages, axe-ages;
Shields are cleft in twain;
Storm-ages, murder-ages;
Till the world falls dead,
And men no longer spare
Or pity one another."

Norse Mythology (R. B. Anderson).

The Fimbul-winter

Seeing that crime was rampant, and all good banished from the earth, the gods realised that the prophecies uttered of old were about to be fulfilled, and that the shadow of Ragnarok, the twilight or dusk of the gods, was already upon them. Sol and Mani grew pale with affright, and drove their chariots tremblingly along their appointed paths, looking back with fear at the pursuing wolves which would shortly overtake and devour them; and as their smiles disappeared the earth grew sad and cold, and the terrible Fimbul-winter began. Then snow fell from the four points of the compass at once, the biting winds swept down from the north, and all the earth was covered with a thick layer of ice.

"Grim Fimbul raged, and o'er the world
Tempestuous winds and snowstorms hurled;
The roaring ocean icebergs ground,
And flung its frozen foam around,
E'en to the top of mountain height;
     No warming air
     Nor radiance fair
Of gentle Summer's soft'ning light,
Tempered this dreadful glacial night."

Valhalla (J. C. Jones).

This severe winter lasted during three whole seasons without a break, and was followed by three others, equally severe, during which all cheer departed from the earth, and the crimes of men increased with fearful rapidity, whilst, in the general struggle for life, the last feelings of humanity and compassion disappeared.

The Wolves Let Loose

In the dim recesses of the Ironwood the giantess Iarnsaxa or Angur-boda diligently fed the wolves Hati, Sköll, and Managarm, the progeny of Fenris, with the marrow of murderers' and adulterers' bones; and such was the prevalence of these vile crimes, that the well-nigh insatiable monsters were never stinted for food. They daily gained strength to pursue Sol and Mani, and finally overtook and devoured them, deluging the earth with blood from their dripping jaws.

"In the east she was seated, that aged woman, in Jarnrid,
And there she nourished the posterity of Fenrir;
He will be the most formidable of all, he
Who, under the form of a monster, will swallow up the moon."

Voluspa (Pfeiffer's tr.).

At this terrible calamity the whole earth trembled and shook, the stars, affrighted, fell from their places, and Loki, Fenris, and Garm, renewing their efforts, rent their chains asunder and rushed forth to take their revenge. At the same moment the dragon Nidhug gnawed through the root of the ash Yggdrasil, which quivered to its topmost bough; the red cock Fialar, perched above Valhalla, loudly crowed an alarm, which was immediately echoed by Gullin-kambi, the rooster in Midgard, and by Hel's dark-red bird in Nifl-heim.

"The gold-combed cock
The gods in Valhal loudly crowed to arms;
The blood-red cock as shrilly summons all
On earth and down beneath it."

Viking Tales of the North (R. B. Anderson).

Heimdall Gives the Alarm

Heimdall, noting these ominous portents and hearing the cock's shrill cry, immediately put the Giallar-horn to his lips and blew the long-expected blast, which was heard throughout the world. At the first sound of this rally Æsir and Einheriar sprang from their golden couches and sallied bravely out of the great hall, armed for the coming fray, and, mounting their impatient steeds, they galloped over the quivering rainbow bridge to the spacious field of Vigrid, where, as Vafthrudnir had predicted long before, the last battle was to take place.

The Terrors of the Sea

The terrible Midgard snake Iörmungandr had been aroused by the general disturbance, and with immense writhings and commotion, whereby the seas were lashed into huge waves such as had never before disturbed the deeps of ocean, he crawled out upon the land, and hastened to join the dread fray, in which he was to play a prominent part.

"In giant wrath the Serpent tossed
In ocean depths, till, free from chain,
He rose upon the foaming main;
Beneath the lashings of his tail,
Seas, mountain high, swelled on the land;
Then, darting mad the waves acrost,
Pouring forth bloody froth like hail,
Spurting with poisoned, venomed breath
Foul, deadly mists o'er all the Earth,
Thro' thundering surge, he sought the strand."

Valhalla (J. C. Jones).

One of the great waves, stirred up by Iörmungandr's struggles, set afloat Nagilfar, the fatal ship, which was constructed entirely out of the nails of those dead folks whose relatives had failed, through the ages, in their duty, having neglected to pare the nails of the deceased, ere they were laid to rest. No sooner was this vessel afloat, than Loki boarded it with the fiery host from Muspells-heim, and steered it boldly over the stormy waters to the place of conflict.

This was not the only vessel bound for Vigrid, however, for out of a thick fog bank towards the north came another ship, steered by Hrym, in which were all the frost giants, armed to the teeth and eager for a conflict with the Æsir, whom they had always hated.

The Terrors of the Underworld

At the same time, Hel, the goddess of death, crept through a crevice in the earth out of her underground home, closely followed by the Hel-hound Garm, the malefactors of her cheerless realm, and the dragon Nidhug, which flew over the battlefield bearing corpses upon his wings.

As soon as he landed, Loki welcomed these reinforcements with joy, and placing himself at their head he marched with them to the fight.

Suddenly the skies were rent asunder, and through the fiery breach rode Surtr with his flaming sword, followed by his sons; and as they rode over the bridge Bifröst, with intent to storm Asgard, the glorious arch sank with a crash beneath their horses' tread.

"Down thro' the fields of air,
With glittering armour fair,
In battle order bright,
They sped while seething flame
From rapid hoofstrokes came.
Leading his gleaming band, rode Surtur,
'Mid the red ranks of raging fire."

Valhalla (J. C. Jones).

The gods knew full well that their end was now near, and that their weakness and lack of foresight placed them under great disadvantages; for Odin had but one eye, Tyr but one hand, and Frey nothing but a stag's horn wherewith to defend himself, instead of his invincible sword. Nevertheless, the Æsir did not show any signs of despair, but, like true battle-gods of the North, they donned their richest attire, and gaily rode to the battlefield, determined to sell their lives as dearly as possible.

While they were mustering their forces, Odin once more rode down to the Urdar fountain, where, under the toppling Yggdrasil, the Norns sat with veiled faces and obstinately silent, their web lying torn at their feet. Once more the father of the gods whispered a mysterious communication to Mimir, after which he remounted Sleipnir and rejoined the waiting host.

The Great Battle

The combatants were now assembled on Vigrid's broad plain. On one side were ranged the stern, calm faces of the Æsir, Vanas, and Einheriar; while on the other were gathered the motley host of Surtr, the grim frost giants, the pale army of Hel, and Loki and his dread followers, Garm, Fenris, and Iörmungandr, the two latter belching forth fire and smoke, and exhaling clouds of noxious, deathly vapours, which filled all heaven and earth with their poisonous breath.

"The years roll on,
The generations pass, the ages grow,
And bring us nearer to the final day
When from the south shall march the fiery band
And cross the bridge of heaven, with Lok for guide,
And Fenris at his heel with broken chain;
While from the east the giant Rymer steers
His ship, and the great serpent makes to land;
And all are marshall'd in one flaming square
Against the Gods, upon the plains of Heaven."

Balder Dead (Matthew Arnold).

All the pent-up antagonism of ages was now let loose in a torrent of hate, each member of the opposing hosts fighting with grim determination, as did our ancestors of old, hand to hand and face to face. With a mighty shock, heard above the roar of battle which filled the universe, Odin and the Fenris wolf came into impetuous contact, while Thor attacked the Midgard snake, and Tyr came to grips with the dog Garm. Frey closed with Surtr, Heimdall with Loki, whom he had defeated once before, and the remainder of the gods and all the Einheriar engaged foes equally worthy of their courage. But, in spite of their daily preparation in the heavenly city, Valhalla's host was doomed to succumb, and Odin was amongst the first of the shining ones to be slain. Not even the high courage and mighty attributes of Allfather could withstand the tide of evil as personified in the Fenris wolf. At each succeeding moment of the struggle its colossal size assumed greater proportions, until finally its wide-open jaws embraced all the space between heaven and earth, and the foul monster rushed furiously upon the father of gods and engulphed him bodily within its horrid maw.

"Fenrir shall with impious tooth
Slay the sire of rolling years:
Vithar shall avenge his fall,
And, struggling with the shaggy wolf,
Shall cleave his cold and gory jaws."

Vafthrudni's-mal (W. Taylor's tr.).

None of the gods could lend Allfather a helping hand at that critical moment, for it was a time of sore trial to all. Frey put forth heroic efforts, but Surtr's flashing sword now dealt him a death-stroke. In his struggle with the arch-enemy, Loki, Heimdall fared better, but his final conquest was dearly bought, for he, too, fell dead. The struggle between Tyr and Garm had the same tragic end, and Thor, after a most terrible encounter with the Midgard snake, and after slaying him with a stroke from Miölnir, staggered back nine paces, and was drowned in the flood of venom which poured from the dying monster's jaws.

"Odin's son goes
With the monster to fight;
Midgard's Veor in his rage
Will slay the worm;
Nine feet will go
Fiörgyn's son,
Bowed by the serpent
Who feared no foe."

Sæmund's Edda (Thorpe's tr.).

Vidar now came rushing from a distant part of the plain to avenge the death of his mighty sire, and the doom foretold fell upon Fenris, whose lower jaw now felt the impress of that shoe which had been reserved for this day. At the same moment Vidar seized the monster's upper jaw with his hands, and with one terrible wrench tore him asunder.

The Devouring Fire

The other gods who took part in the fray, and all the Einheriar having now perished, Surtr suddenly flung his fiery brands over heaven, earth, and the nine kingdoms of Hel. The raging flames enveloped the massive stem of the world ash Yggdrasil, and reached the golden palaces of the gods, which were utterly consumed. The vegetation upon earth was likewise destroyed, and the fervent heat made all the waters seethe and boil.

"Fire's breath assails
The all-nourishing tree,
Towering fire plays
Against heaven itself."

Sæmund's Edda (Thorpe's tr.).

The great conflagration raged fiercely until everything was consumed, when the earth, blackened and scarred, slowly sank beneath the boiling waves of the sea. Ragnarok had indeed come; the world tragedy was over, the divine actors were slain, and chaos seemed to have resumed its former sway. But as in a play, after the principals are slain and the curtain has fallen, the audience still looks for the favourites to appear and make their bow, so the ancient Northern races fancied that, all evil having perished in Surtr's flames, from the general ruin goodness would rise, to resume its sway over the earth, and that some of the gods would return to dwell in heaven for ever.

"All evil
Dies there an endless death, while goodness riseth
From that great world-fire, purified at last,
To a life far higher, better, nobler than the past.

Viking Tales of the North (R. B. Anderson).