This was the original intended ending to Soul Reaver 1by Mischief Maker 06/21/2012, 6:18am PDT
Legal shit and other problems lead to Soul Reaver 1 being pared down to half its intended length and ending the story on a cliffhanger that sadly slipped off the cliff in Soul Reaver 2. However, to avoid bugging up their final build, many completed levels, abilities, and other assets from the intended game were still on the disc and uncovered by hackers, including the full dialouge.
Here's the audio from the cut bossfight against your remaining brother Turiel, this audio was partially used for a fight against a regular enemy who just happened to have a super duper power-granting soul:
Here's the audio from the original ending, where after killing Kain you gain the ultimate Reaver that can instakill any vampire. You return to the gigantic pipe organ cathedral that was humanity's failed last attempt at wiping out the vampires and finally put it to use.
There's more, but the original article that outlined it all seems to be down at the moment.