Tome unlock for Stone Troll in Black Crag. Enter the Despair Pits (2.97k, 27.79k) (middle of W border). In the N end of the cave, there's something on the ground that you can interact with called Drul's Lunch. Interacting with it gives you a text saying you could poison. Go to the S end of the cave & kill Skittering Spiders to get a Poison Gland. Then interact with Drul's Lunch to get the unlock. Drul's Lunch is a pile of 4 rocks on the ground located at 7500,20290.
Do note you can't do this with more than one poison gland so destroy the extra ones.