Zoltun Kulle is a tome found in Act II of Diablo III, found at the beginning of the "Betrayer of the Horadrim" quest.
Written by Adria, it reads as follows:
Murderer. Torturer. Monster. These were the titles bestowed upon Zoltun Kulle by the Horadrim. Though it cost them dearly, they killed the wizard for his crimes against nature and sealed his broken corpse within a shadow realm. It seems they were afraid he might return.