Evil in the House of Dr. Fleshenstein is a horde mode extra content unlocked after beating the main game. In order to play it, you need to go to the Main Menu and select “Extras”.
Part 1 – Digits
1) You’ll need to find three numbers scattered around the house.
These numbers are “6”, “7”, and “8”. They are all randomly generated, except for “7”, which is always found on the ground floor next to an oval table.
“6” seems to most commonly spawn on the ground floor too, but can also spawn on the second floor.
“8” always spawned on the second floor in all of my playthroughs.
After finding all digits and interacting with them (you need to left click them), you’ll need to return to the living room where you spawned and turn on the generator. Then comes part two:
Part 2 – Mannequin items
2) You’ll need to give each of the three mannequins their items.
You’ll find the Mannequin room after opening the doors that were previously locked before turning on the generator, on the ground floor.
Crucifix: You’ll find the crucifix on the outside part of the second floor, where there is a big river of blood. Make sure to pick it up by interacting with it.
Cube with number “5”: You’ll find it on a very narrow hallway on the second floor. To get there, take a right turn right after you ascend the stairs and keep going straight – can’t miss it.
Peepaw shirt: You’ll find it on the ground floor, in the first room to the left of where you spawn.
Once you pick up all the three items, you need to head back to the Mannequin room and interact with each of them. After that, a door will open.
Part 3 – The Basement
You must pay a lot of attention during this part, since some zombies will spawn very quickly and will give you a very short reaction time. Make sure your weapons are fully loaded and that you’re paying close attention to each and every single place you go.
You’ll need to find a Lighter and a bundle of Dynamite. They are both in the basement, in opposite sides.
After you gather the two, head to the pile of rocks and interact with it.
After the explosion, head to the room and the right and be careful: there will be three zombies lurking there. Take your time and kill them one by one. If you stick to the right side, a passage will open – follow it and you’ll arrive at the final part.
Part 4 – Dr. Fleshenstein Boss Fight
There will be several red portals where Dr. Fleshenstein’s vessel will spawn, so make sure to find a comfortable position that makes sense and that will cover all possible angles.
Throughout this fight, you’ll need to shoot Dr. Fleshenstein’s brain – after taking a hit, the brain will teleport to somewhere else in the room. It’s very simple: once you shoot the vessel, take the time to find the brain and shoot it once. Do this until he’s dead.
(The brain is not the literal brain of the zombie, it’s a separate unit that will be on the floor).