The last boss fight of the game would have taken place in Kain's Mountain Retreat, the last Kain's hiding place. Kain, expecting Raziel, encouraged and taunted Raziel to kill him if he could muster the courage to do it. Raziel eventually emerged victorious after using Ariel's soul as an enhancing element. By striking Kain down, Raziel would have obtained the red and black Reaver, known as the Amplified Reaver, encompassing any other enhancement he had before. Raziel sarcastically noted that both Kain's and Ariel's souls were now festering and bound together within the wraithblade for eternity afterwards.
There is little known about the boss fight, but it is very likely the course would have been the same as the previous fights with Kain. He would have been appearing in several places around the main chamber and Raziel would have to strike him three times in order to kill him. The main chamber has a large Kain symbol on the floor and staircase, possibly leading to Kain's throne.
This scene was later re-enacted in a similar fashion in Defiance, where the two fought and Raziel succeeded in defeating Kain.