I took the quest from him, but before I found the Troll King I picked up some Tenebrium and the log updated telling me I'd found some ore in the Troll king's cave even though I hadn't been there.
Eventually made it to the cave, cleared it out, picked up some ore, but speaking with Brandon there's no option to hand him the ore, and it doesn't show up in my pack to trade it (even if I remove it from the safety box).
