Step by step for how to unlock it yourself. But first, I'll make it clear that you MUST be playing on original difficulty to unlock this. You CANNOT unlock it on easy difficulty:
- Acquire the "hell's retriever" tomahawk/tactical weapon by "feeding" the three hounds.
- Optional (but much easier than wasting money on the box) - Use the tomahawk to retrieve the 5 hidden skulls around the map. Doing this spawns a free blundergat on the warden's desk.
- Retrieve and assemble the Acid Gat parts at the bench next to the electric cherry perk. Upgrade the blundergat to the acid gat.
- Retrieve the parts for the plane and head to the bridge. PaP the Acid Gat. This gives you the "Vitriolic Withering", which you'll need for the last step.
- After returning from the bridge, head to the cell next to the warden's office with the poster on the back wall. Use the tomahawk to destroy the poster, revealing an afterlife symbol. Use the afterlife panel near the warden's office to enter the new area to the left. Inside on the floor is a skeleton buried in dirt. To the right is a spoon. Zap it with lightning.
- Head to the back of the cafeteria and use the tomahawk to grab the spoon on the table behind the back window to the left.
- Head to the infirmary, to the room with the single bathtub filled with blood. Press X next to the tub. You'll see a spoon stir the blood.
- Go to the showers, and use the Vitriolic Withering to kill, kill, kill. Once you've killed enough zombies (usually around 50 does it), you'll hear the audio cue (samantha laughing).
^^^ Two things, one, it can be the acid gat in the shower part, you do not have to PaP it, although if you have the acid gat, I'd recommend PaP'ing anyway. Two, you may not hear the sound queue, which I believe it should be a laugh. After you have about 50 kills, if you don't hear anything, go check the tub anyway.