First, talk to the Council of Loathing and they will tell you to go slay the Boss Bat. Go to the Nearby Plains and go down the Bat Hole. Next, kill bats till you get a Fresh-Pine Air Freshener.
Then, adventure into the Guano Junction and kill the bats till you receive three Sonar-In-A-Biscuit's and use them in your inventory.
It will say you hear the sound of crumbling walls and each Sonar-in-a-Biscuit unlocks a part in the Bat Hole.
The last one unlocked is the Boss Bat's Lair where you will have to kill the Boss Bat.
After you beat him you will acquire Dense Meat Stack and Boss Bat Bandanna. Return to The Council of Loathing and they will give you a Batskin Belt. Now you have completed the quest.
