This, like the Ariel Reaver, was taken out of the game due to time constraints.
Originaly concieved to be the combination of Kain and Ariel's souls into
the regular Reaver,
it was originally intended for the last portion of the story,
and to be the greatest weapon in the game (not counting the deleted Armoury,
but that's another story)
It has the same powers as the Soul Reaver and looks Jet Black with a red
aura swirling around it.
In one of the star portol things just before your final fight with Kain it
shows Raziel with the Kain Reaver
standing in what looks like an end of a hallway that ends at a wall.
The Kain/Amplified Reaver returns to the Soul Reaver when you side-step.
Learn more about the Kain/Amplified Reaver here