In the time of Vorador, centuries before Kain was made, the Sarafan warrior priests waged a merciless war against the vampire tribes of Nosgoth.  Emboldened by righteousness, they committed unspeakable and indiscriminate acts of violence - massacring fledglings and ancients alike, they decimated entire bloodlines in mere decades.  Now their husks lay here - murderers enshrined.

As Raziel steps into the ancient hallway leading to the Sarafan tomb, the Elder God warns:

Elder God:
Take heed, Raziel.  A forgotten history lies within.  Know thyself - though it may destroy you...

Having discovered a twisting passageway, Raziel makes his way into the depths of the tomb.  The passage apparently dead-ends, blocked by a massive stone that seals the tomb.  The stone is no impediment to Raziel, and he easily pulls it free.
 

Raziel: As I pulled the stone free, a sigh of sepulchral air escaped the inner chamber.  I was not prepared for what lay beyond this threshold... These crypts... defiled caskets of Sarafan saints... bearing my brothers’ names...  And my own... The irony of Kain’s blasphemous act rushed in on me with the crushing force of revelation... Were my hands not as bloody as these?  Worse, I had spilled the blood of my brothers - these very comrades whose tombs lay ravaged before me.

Elder God:
Yes, Raziel - you were Sarafan... born of the same force that all but destroyed your race.  Before the dawn of the Empire, you were chosen.

Kain -- Nosgoth’s solitary, self-declared monarch -- plundered this tomb and raised you from these crypts.  Breathing his vampiric gift into your defiled corpses, he resurrected you as his favored sons.
 
 

Tomb Guardian

Raziel: Such loyalty... to one who has you guarding this outpost like a chained dog.  Do you prosper on the scraps he casts you? Tomb Guaridan: Your insults will do nothing to blunt the agonies of your demise. Raziel: Kain killed me once -- behold the result.  I have no more to fear from you. After Raziel defeats the Tomb Guardian, and receives the Force projectile ability:

Elder God:
This relic has infused you with the power to compress and manipulate space.  As your symbiotic weapon, the Soul Reaver is also thus enhanced.  You may focus and project an orb of kinetic energy to strike objects that are otherwise beyond your reach.

If Raziel returns to the Elder God's lair for a hint to locating the Drowned Abbey:

Elder God:
North of the Sarafan Tomb, your brother Rahab has secluded himself and his progeny in the ruins of a half-drowned abbey.

If he visits Ariel before the Drowned Abbey:

Ariel:
North of the Sarafan tomb, a desecrated abbey languishes, half-drowned.

The Drowned Abbey

When Raziel discovers the courtyard of the Drowned Abbey, the Elder God offers the following information: Elder God: Once a sanctuary against the vampire menace, this abbey has been drowned by the deluge spilling from this wounded land.  Your brother Rahab and his brood, devastated even by the feeble rays of Nosgoth’s sun, overcame their vulnerability to water and retreated from the surface.  Now they haunt these ruins, and glide in the darkness of its stagnant depths.

Rahab

Rahab:
Raziel.
 

Raziel: Rahab.  You have adapted well to your environment, for one so maladjusted... Rahab: Do not mock me, Raziel.  You, of all of us, should respect the power bestowed by a limitation overcome. Kain said you would come. Raziel: You speak with the murderer? Rahab: You would do well to mind your blasphemous tongue. Raziel: What more did he tell you? Rahab: That you would destroy me.

Raziel:
I will, indeed.  But tell me, before I tear your soul from its moorings, do you know what we were before Kain spawned us?

Rahab:
Human.

Raziel:
Sarafan, Rahab.  The antithesis of all we ever believed.

Rahab:
Does it matter?  We were lost.  He saved us.

Raziel:
Saved us?  From what?

Rahab:
From ourselves.

If Raziel is defeated in the battle, then upon his return: 

Rahab:
Raziel returns - do you still think you can best me?

When Rahab is destroyed:

Rahab:
I am undone!

Elder God:
Infused with Rahab’s soul, you have overcome your former vulnerability to water’s touch.  Immersion in water will no longer dissolve your physical body, enabling you to swim to areas heretofore beyond your reach.
 

As Raziel exits Rahab’s chapter house, the Elder God offers the following information:

Elder God:
Centuries past, your brother Dumah retreated with his clan into Nosgoth’s northern wilderness.  Empowered by the gift of Rahab’s soul, seek out his mountain stronghold.  But tread carefully - these frozen wastes have mysteries yet to disclose.

If Raziel revisits the Elder God in the Underworld before advancing to the Ruined City of the Dumahim, he receives the following clue:

Elder God:
Beyond the maelstrom of the abyss, a subterranean tunnel leads northward, to the clan territory of your brother Dumah.
Endowed with Rahab’s soul, you may swim to gain entry where your progress was previously impeded. 

If Raziel visits Ariel before proceeding to the Ruined Dumahim city, she offers the following clue:

Ariel:
Beyond the cliffs that witnessed your execution, a silent city sits in frozen tableau.  Locked in eternal limbo, they await redemption, or release. 

The Ruined City of the Dumahim

Raziel: This city once teemed with the life of my kind.  Could it be that other clans had suffered the same fate as mine? In his madness, could Kain spare none of his brood? Elder God: It was not Kain, but Dumah’s own arrogance that brought the downfall of his clan.  These are human weapons, Raziel - believing themselves invincible, Dumah and his offspring failed to see an attack coming from the least likely assailants...  Complacent in their arrogance, they were taken by surprise, allowing the human vampire hunters to decimate their ranks with little resistance.  The few that escaped have been reduced to scavengers.

Dumah

After working his way through the city, Raziel finally steps into Dumah’s throne room.  The corpse of Dumah slouches on its throne, impaled by multiple spears. Raziel: My brother, Dumah - a powerful warrior, in life.  He would have burned with shame, to have me find him here like a stuck pig. If Raziel goes into the Spectral plane before resserecting Dumah, he encounters his spirit haunting the thone room:

Dumah:
Come to save my soul, prodigal brother?

Raziel:
Not to save it, but to take it.  Your only salvation is release.

After Raziel has revived Dumah:

Dumah:
Unbound at last... I thank you, brother. 

Raziel:
Your thanks are premature, Dumah.  I have not forgotten whose hands bore me into the abyss.

Dumah:
The centuries in limbo have honed my strength.  Not even Kain is my equal. 

Raziel:
Even the strongest vampire is vulnerable. 

Dumah:
We shall test your thesis, Raziel. 

Raziel:
My blood-thirst has been superceded by an even darker hunger.  I will consume your soul before this day is done. 

If Raziel is defeated in the battle, then upon his return: 

Dumah:
You are persistent, Raziel.

Upon Dumah’s defeat, and as Raziel receives his soul:

Elder God:
Consuming Dumah’s soul empowers you to wind a constricting band of spectral energy around your enemies.  This energy manifests itself in both the spectral and material realms - and in the physical world, it can be employed to manipulate otherwise immovable objects.

After Raziel defeats Dumah, if he revisits the Elder God in the Underworld before advancing to the Oracle’s Cave, he receives the following clue:

Elder God:
In the furthermost canyon of Nosgoth’s frozen wastes, the hushed silences embrace an ancient enigma.  In the bowels of this labyrinth, your master awaits...

If Raziel visits Ariel before going to the Oracle's Cave:

Ariel:
In the bowels of an inverted clockwork, the tormentor awaits.  Be wary, Raziel - those blind with rage are by destiny ensnared.