Checking the boss's corpse, you realize you won't even need to pick the lock as he has the key! You use it on the chest and find within:
a silver bracelet with a fish pattern worth 5 gp, a very old map that depicts some strange-looking dungeon, a smoking sword with a dwarven face carved into the hilt, a wand of heal, 2 healing potions, and a hat of disguise.