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.
Not much information is known regarding the boss fight in Kain´s mountain retreat. The alpha versions revealed it would have taken place in a room labelled as "Retreat 3" but the room is empty with Kain absent. It can be assumed that the boss fight would have worked similarly to the previous encounters at the Pillars and the Chronoplast, with Kain teleporting around the room and Raziel finding the opportune moment to strike. After defeating Kain, Raziel would have used his Reaver imbued with Ariel´s spirit and bound Kain´s soul together with hers, creating the Amplified Reaver.
The Chronoplast Visions showed a third and final confrontation between Raziel and Kain which appeared to be this boss battle. Exploration of these rooms in betas reveals that these are mostly mock ups repurposed from existing assets and none of the rooms appear in their respective areas in the alphas. Kain's monie room seems to be an exception - although not featured in the alpha it is unusually complex for an area that is barely seen and is covered in shadow in the vision - potentially suggesting more was indeed planned for the area. Ultimately it is unknown whether the movie room was a discarded part of the retreat or simply a mock up like the other vision rooms.
This scene was later re-enacted in a similar fashion in Defiance, where the two fought and Raziel succeeded in defeating Kain.