Upon revisiting Caterpillar's Plot after the battle with the Jabberwock, Alice found Humpty Dumpty sitting near the brick gate that led to the oracle. He occasionally gestured toward a brick that was sticking out and after pressing this brick, it unlocked a secret passage in the gate, where the Blunderbuss was found.
Finally, during the final battle with the Queen of Hearts in Heart of Darkness, Alice found the Blunderbuss on a secret "shelf" beneath one of the larger islands of the Queen. There was also a teleport plate that immediately returned her to the top of the island.
The Blunderbuss only has a single primary attack and one of the only weapons which drains all of Alice's will in a single shot. It unleashes a powerful blast against her enemies that causes a large area of damage. It can be used multiple times as long as Alice has full willpower.
The Blunderbuss is an extremely large weapon, causing Alice to carry it using both hands. The general trigger area is quite petite and thin, however, as it goes further up, the gun's barrel is extremely crooked, and the muzzle fans out to an extreme size. The gun's barrel is primarily brass, while the stock is wooden.
The Blunderbuss is not a key component to the plot of the game as it is a secret. Alice can go through her journey without ever taking notice of it, however, Alice has two opportunities to receive the Blunderbuss during the course of the game.
The Blunderbuss was a cavalry weapon which was primarily used during the seventeenth century in countries such as England, France and also, very commonly, Mexico. It is considered to be a very early version of the Shotgun. It is also known to have a handgun-like counterpart known as a "Dragon", or "Dragoon".
It's effective use is feasible mostly on lower difficulties, and only for fun in higher, as it tends to drain Alice's whole Will for one single burst, leaving her unable to fight back if there are other foes still standing strong in the vicinity.