A Trail of Pure Vile Evil is a Tome of Knowledge Bestiary entry for Plaguebeast of Nurgle.

Instructions: Order & Destruction

Not implemented yet

The mechanic is a little complicated:

Find at 20.5k 13.8k in Ostland a big altar of Nurgle with 'Plaguesworn Nobles' (zealots) and Nurglings around.
Kill one of the 4 'Plaguesworn Nobles' (zealots).
Wait for him to respawn. When he does, the four will start a ritual that makes spawn a 'Noxious Plaguebeast' (Champion).
Kill the 'Noxious Plaguebeast'. He may drop an item that looks like a potion in the inventory (drop-rate seems to be in between 25% and 50%). That's this item that is needed for the unlock.
If he did not drop the item, you can immediatly kill once again the 'Plaguesworn Nobles', and restart the ritual until the item drops.
Once you have the item needed give it:

For Order: Surely to Brenard the Alchemist in Felde Castle (Empire Chapter 6) in Troll Country.
For Destruction: To Voxanna the Transmuter in Felde (Chaos Chapter 6) in Troll Country.

Rewards