This is a meta quest that you will receive when you approach any of the cellars where blood is stored on Andrus's orders for the first time.
Ambrus's soldiers have set up outposts all throughout his fiefdom. What are they hiding in them?
Ambrus's Cellars
Quest stages of Ambrus's Cellars
1. Investigate the outposts of Ambrus's soldiers
Ambrus set up heavily defended outposts throughout his fiefdom. Maybe I should check what's going on in there?
Eliminate all the soldiers you encounter...
... then retrieve the orders issued by Ambrus from the corpse of one of them.
Three cellar-related quests will appear in your journal:
2. Ending
Ambrus's men store barrels of human blood, intended for later use by their vrakhiri masters. I cannot allow this to continue.