Step 1) Survive the boss round by standing on the apothicon markings which spawn on the floor and killing zombies ( they only spawn when you're on the marking ).

Step 2) Claim your reward at the table to the right of the mystery box.

Step 3) Place the fuse in the fuse box in the showers on the right side wall.

Step 4) Complete all three lockdowns (Gamma, Sigma and Omicron) by activating the respective buttons in the three rooms and surviving.

Step 5) Now the buyable ending at the electric chair is buyable.

Kind of hidden feature:

Buyable Juggernog from the crate in the crypt.