While killing bats as a rat.
I returned fully armored with 3 mercenaries.
While the bat spawns inside the cave, it can wander between zones, which is pretty cool. It can also drop a fang which crafts a "potion of recovery", which apparently heals a player's health to full. That would be very powerful in the hands of a top-ten player, and could be a good income source for mid-levels.