The protagonist resurrects the soul of the traitor Horadrim, Zoltun Kulle, in order to recover the Black Soulstone, who reveals that it is hidden in his archives.
The protagonist resurrects the soul of the traitor Horadrim. This is to recover the Black Soulstone. Zoltun Kulle is the traitor Horadrim. It is hidden in his archives. This is what the Black Soulstone reveals.