Reward: Hobbe Leg
Go to the Temple of Light and speak to err, Tommy? Anyway, he asks for you to clear his cave (home) from Hobbes, not something we haven't done before though, right? Anyway, go to Echo Mine where you will be greeted by a swarm of Hobbes outside.
You only need to kill fifteen stumbling Hobbes, and there are diary pages spread throughout the cave you may pickup if you so choose. After you kill the fifteen Hobbes you'll have to go kill their leader. He'll summon a couple more enemies but they're all easy kills, take the diary page behind him if you'd like and open the chest. Go back to the cave entrance and you'll find out that Tommy is more of a crazy asshole than you originally thought. He wanted you to be killed, you'll have to fight Tommy now, it only took me one hit so I imagine nobody will have any troubles with him. Take his key and leave the mine.