Brianna in Thundermist Village offered to take me on as an apprentice and teach me the ways of her craft. But it seems that I am just doing her chores for her. She asked me to get tough rumbler hides that have never been exposed to light. I should seek out a tunnel where rumblers live. Brianna told me that the caves can be found in The Dead River Basin, between Thundermist Village and the giant controlled bridge.