The Fire Reaver is obtained by finding the Fire Forge and passing your
Soul/Kain/Ariel Reaver over it.
From then on in any future point in the game just pass your Reaver
over any open flame to turn it into the Fire Reaver.
Also this looks different and has different abilities, when you deal
the final blow to an enemy (Triangle) it burns them to death rather than
exploding them.
Combined with the Projectile ability the Fire Reaver throws Fire-balls
(My favorite move!)
The Fire Rever turns to the normal Soul Reaver when you jump in water
or Side-Step.
The Soul Reaver is unable to turn to the Fire Reaver in the Dead realm.