Vidian, Lord of Envy is a tome that can be found in the Sanctum of Blood in the Temple of the Firstborn. It drops from a nephalem corpse in the chamber soon after fighting and defeating Vidian. It was written by Tyrael.
It reads as follows:
"Vidian was always an...unusual demon. I only beheld him once, as he would seldom obey the will of his master, Azmodan. But on that occasion, he was only too ready to bring disorder to the angelic host. When he first appeared, we did not know of his insidious threat, and focused our attacks on his mightier brethren. It did not take long before the angels themselves were beginning to turn on each other, and question their leaders. By the time we figured out what had happened, the battle was lost, and it was too late."