This is a Tome unlock for Plaguebeast fixed in Patch 1.4.3 (2011)
The mechanic is complicated. First of all, 20k 15k in Ostland is the place where you have to be. The name of the mob that has a chance to drop the item is Noxious Plaguebeast (Champion).
To spawn the Plaguebeast, kill one of the four zealots. When this zealot respawns, the four will start the ritual that spawns the Champion. Kill him to drop the item. You can kill the zealot immediatly after having killed the Plaguebeast if he has not dropped the item, to do the ritual until you have it.
The drop-rate for the item (looks like a potion in your inventory) seems to be in between 25% and 50%.
After having dropped the item, give it to Voxanna The Transmuter, at Chap 6 Chaos WC.