The Bells of Awakening
According to the prophecy,
[Those] who art Undead, art chosen…
In [their] exodus from the Undead Asylum, maketh pilgrimage to the land of Ancient Lords…
When [they] ringeth the Bell of Awakening, the fate of the Undead [they] shalt know.
Crestfallen Warrior dialogue:
There are actually two Bells of Awakening.
One's up above, in the Undead Church. The other is far, far below, in the ruins at the base of Blighttown.
Ring them both, and something happens… Brilliant, right?
Not much to go on, but I have a feeling that won't stop you.
So, off you go. It is why you came, isn't it? To this accursed land of the Undead?
Hah hah hah hah…
The role of the protagonist. This title is a reference to a prophecy concerning the end of the Age of Fire. It states that at the very end of the Age of Fire, an Undead champion will appear and undertake a pilgrimage that will decide the fate of the world.
After ringing the two Bells of Awakening, the Undead is tasked with acquiring the Lordvessel from Anor Londo, the city of the gods. With this artifact, the Undead must then decide to either succeed Lord Gwyn and extend the Age of Fire, prolonging the rule of Gods or usher in the Age of Dark and the rule of man.
The world was once unformed and the Everlasting Dragons, unliving beings, held sole dominion over all. However, the First Fire was ignited and within the flames were found powerful souls.
Each Lord Soul grants its bearer specific and unique characteristics. The finders of these souls, having been granted great power, became godlike entities:
Nito, the First of the Dead; the Witch of Izalith and her Daughters of Chaos; Gwyn, the Lord of Sunlight; and the furtive Pygmy, "so easily forgotten".
The Pygmy, whose Lord Soul is known as the Dark Soul, dispersed some of his power in order to create the human race; each human bears a minuscule piece of the Dark Soul in the form of a fragment of Humanity.
With the advent of the First Flame, the four Lords challenged the Dragons for unknown reasons, perhaps for control of the world. During the war, Seath the Scaleless betrayed his own kind, leading Gwyn to use his powerful lightning bolts to peel away the stone scales of the Dragons. The Witches of Izalith conjured powerful firestorms and Nito unleashed a "miasma of death and disease" and the //"dragons were no more". Thus began the rule of Gwyn and the Age of Fire.
The First Flame started to fade at one point and the Age of Dark was nigh.
The Witch of Izalith ambitiously attempted to recreate the First Flame with her Lord Soul but instead spawned the Flame of Chaos. When she failed to control the new flame, it consumed her and molded her into a twisted being known as The Bed of Chaos, the source of all demons. Most of her followers and children were also consumed by the Chaos Flame and were similarly transformed.
In a last effort to preserve the Age of Fire, Gwyn ventured from the city of Anor Londo to the Kiln of the First Flame with some of his faithful knights and there sacrificed himself to the fire, hoping to rekindle it, however lacking all the Lord Souls, Gwyn's sacrifice was not enough and only "embers remained".
Concurrently, a curse appeared among humanity. Those cursed with Undeath were marked with the Darksign. Its bearers will be revived after death but they will lose their Humanity with each death. Those who lose all sanity eventually become Hollow, effectively losing control over their body and will. As such, over time, many Undead have acquired a variety of ways to either collect or steal Humanity from others, like the Darkwraiths who hunt down and drain Humanity from innocents.
Those cursed are brought to the Northern Undead Asylum, where they are locked away from the rest of the world, to "await the end of the world".
The protagonist wakes locked up in such a cell, but a nameless knight helps them escape. This knight then speaks of a prophecy:
Thou who art Undead, art chosen…
In thine exodus from the Undead Asylum, maketh pilgrimage to the land of Ancient Lords…
When thou ringeth the Bell of Awakening, the fate of the Undead thou shalt know.
Once the Chosen Undead has rung the Bells of Awakening, the Primordial Serpent Kingseeker Frampt, is awoken. He directs the Chosen Undead to reacquire the Lord Souls and succeed Lord Gwyn by defeating him and linking the Flame, thus undoing the curse. To complete this task, the Chosen Undead must first retrieve the Lordvessel from Anor Londo, the forsaken city of the gods:
Chosen Undead. Your fate is… to succeed the Great Lord Gwyn.
So that you may link the Fire, cast away the Dark, and undo the curse of the Undead.
To this end, you must visit Anor Londo, and acquire the Lordvessel.
Those who seek the Realm of the Lords must brave Sen's Fortress, a deadly house of traps.
Many have gone before you, but none have returned.
Fate has chosen you, but proceed with caution.
As Kingseeker, I shall now instruct you, the Lord's successor in your next task.
To achieve your fate, fill the vessel with powerful souls,
commensurate to the great soul of Gwyn.
Scarce few possess such brilliant souls.
Gravelord Nito, the Witch of Izalith,
the Four Kings of New Londo, who inherited the shards of Gwyn's soul…
and Lord Gwyn's former confidant, Seath the Scaleless.
All of their souls are required to satiate the lordvessel.
If the Chosen Undead refrains from placing the Lordvessel on the altar, they can instead meet Darkstalker Kaathe in the Abyss, another Primordial Serpent who opposes Frampt. He tells a different prophecy:
Prove you must, that the truth becomes you.
Seek Anor Londo.
And claim Gwyn's heirloom, the Lordvessel.
Now go, Undead warrior.
Show the world that the truth becomes you.
The truth I shall share without sentiment.
After the advent of fire, the ancient lords found the three souls.
But your progenitor found a fourth, unique soul.
The Dark Soul.
Your ancestor claimed the Dark Soul and waited for Fire to subside.
And soon, the flames did fade, and only Dark remained.
Thus began the age of men, the Age of Dark.
Lord Gwyn trembled at the Dark.
Clinging to his Age of Fire, and in dire fear of humans,
and the Dark Lord who would one day be born amongst them,
Lord Gwyn resisted the course of nature.
By sacrificing himself to link the fire, and commanding his children to shepherd the humans,
Gwyn has blurred your past, to prevent the birth of the Dark Lord.
I am the primordial serpent.
I seek to right the wrongs of the past to discover our true Lord.
But the other serpent, Frampt, lost his sense, and befriended Lord Gwyn.
Undead warrior, we stand at the crossroad.
Only I know the truth about your fate.
You must destroy the fading Lord Gwyn, who has coddled Fire and resisted nature,
and become the Fourth Lord, so that you may usher in the Age of Dark!
Kaathe offers a different path to end the Age of Fire and usher in the Age of Dark instead. He implies that the prophecy of the Chosen Undead is a lie, however upon careful examination of facts, it becomes apparent that both Frampt and Kaathe have been feeding the Chosen Undead half-truths in order to advance their own agendas. Indeed, there may be no prophecy at all and the Undead may be no more than one pawn among many in a greater game. The Chosen Undead must ultimately come to their own conclusions about which path will save the world and which might condemn it.
Once the Lordvessel is placed, the Chosen Undead must reacquire the Lord Souls: one from Seath the Scaleless, one from the Four Kings, one from Gravelord Nito and one from the Bed of Chaos.
Inevitably, the Chosen Undead must fight and defeat Gwyn. Once this is accomplished, a choice is presented: rekindle the Flame, thus prolonging the Age of Fire and the rule of existing deities by sacrificing their life to the Fire; or let the Flame wither and die, ushering in the Age of Dark where humans would rule.
Why The Chosen Undead is a Fitting Heir to the Flame
In the cutscene when the Furtive Pygmy finds the Dark Soul, the flames have died down somewhat, which might be because the Dark Soul is the fuel for the flames. This might be why one can spend humanity to increase the power of a bonfire.
Gwyn was not a fitting heir to inherit the flames because his soul did not contain a fragment of the Dark Soul. The gods are beings of Light and fear the Dark.
When Gwyn's sacrifice didn't work, Gwyndolin must have understood that only those who had a piece of the Dark Soul in them could inherit the flame. Perhaps this was explained to him by the Primordial Serpent Frampt (as the Primordial Serpents seem to have a good understanding of the souls and are heavily involved).