In the heart of Blackrock Spire you will find Mother Smolderweb. Engage her and get her to poison you. Chances are good that you will have to kill her as well. Return to Ragged John when you are poisoned so that he can 'milk' you.
It's probably one of the most potent poisons in the world, [race]!
I've overheard the Blackrock speaking about its lethality in my jaunts into the Spire. They call it 'Mother's Milk', named after Mother Smolderweb, the big nasty spider in the Skitterweb cave.
Now here's what I need from you: A live sample of the Milk, flowing through YOUR veins. Just bringing me the gland won't be enough! Get her to poison you and come back to me!
Then... I get to milk ya!
<Rubs his hands together in anticipation.>