Wood from a Mountain Ash is the bane of the Were-bear in my game. It comes from a legend in Scandinavia, where his family (of theriomorphs) is from. Apparently, the locals were told to plant them outside their homes to keep werewolves at bay. He's susceptible to weapons made from the wood, as well as treating the entire plant as highly toxic if ingested (such as a brewed tea from the leaves...)