The Discovery of Sanctuary is a trio of tomes found in the Battlefields of Eternity within Act V of Diablo III. They are set prior to the Sin War, with each of the Prime Evils reacting to the theft of the Worldstone.
They read as follows:
Cursed am I to lead an army of the blind. They do not perceive that the angels are fleeing this realm, and the ones they find are merely trapped or lost. A great change is upon us. Withdraw from the fields, my brothers. Some battles can only be won with words.
Enough of your idle speculation, Mephisto! I breached the fortress and saw it firsthand. The Worldstone is gone! The angels I killed knew nothing about it. But since you are so perceptive, maybe you remember who else has been missing: Lilith. We must find her, rip her limb from limb, take the Worldstone back!
You are all deceived, my brothers. A new age has already begun. Can you not sense them? Ugly creatures, born in shadow, they feel terror, hatred, and the desire to destroy. Yes... but they are not ours yet. But they will open their world to us very soon. An invitation we cannot refuse.