Malthael's Plan is a tome found in Act V of Diablo III. It is written by Tyrael, and it is dropped by him just before boarding the ram in Breaching the Fortress. Obtaining the journal will add to the Field Research achievement.
It reads as follows:
Malthael is fundamentally altering the Black Soulstone. It will now pull in all the demonic essence from whatever plane of existence it finds itself in. But mankind is born of angel and demon alike. Every mortal save myself has demonic essence as an intrinsic part of their being. Losing that will rip their body and soul apart.