Mithril is not especially harmful to dragons, you know. The lore of mithril is that it's as tough as dragon scales when forged into an armor, which is its most common use. It's not made into weapons that often because it's much better suited as an armor than a weapon. Remember Frodo's mithril coat in Lord of the Rings? That's what mithril's like.

But he won't fight unless provoked. He's almost strictly an interaction character.

By the way, could I make an evil Rider with an Ancalagon-sized dragon as a companion?