Chapter 1:
Letter from Mom (inside the house next to stairs)
Invoice (inside the house upstairs)
Cipher of parchment (in the attic)

Chapter 3:
Localizer Manual (in a shed on the localizer)

Chapter 5:
Decrypted telegram (in the house on the working table)
A threat note (on the fence right where you scan a person jumping over it )
A photo of Emma (in a tree house before chase scene)
A burnt fragment of a leaflet (inside charred inn ruin)
A page from the Bible (in the room with a safe after you fell down with it)

Chapter 7:
Notes from the camp (in a tent camp)
Map (in a tent camp)
First note from H. (Hidden chest - E,4 Map)
Second note from H. (Hidden chest - D,9 Map)
Warning message (in the house - enigma working room - right part of a working table)
Third note from H. (Hidden chest - G,7 Map)
Fourth note from H. (Hidden chest - K,5 Map)
Fifth note from H. ( Hidden chest - H,2 Map)
Observation note on Adam and Emma (inside the spy house)
A photo of a car (Inside the spy house)
Secret notes of a Soviet spy (inside the spy house)
Notes on Adam and Emma (inside the spy house)

Chapter 9:
Photo of Emma with a threat (given automatically after you meet Emma)
Old police report (inside the sherrif's car)
Sherrif's photo with his family (inside the sherrif's car - sun visor)
Police report (inside the sherrif's car)

Chapter 2:
Photo at the mine (table in the dinning room - big clock room)
Land pre-emption contract (working table next to the dinning room - blocked by armchair)
Inn photo (upstairs, end of the hallway, near the window)
Second newspaper clipping (in the shed, next to the door)
Inn advertisement (in the basement)

Chapter 4:
Letter from Maria (upstairs, your father's room)
The first article about the accident (your father's working room once you fall through ceiling)

Chapter 6:
A leaflet about the judge (notice board at the beginning)
A page from bible (in the room 111, to the left before you follow the ghost with candles upstairs)
A letter from one of the guests (on the table at room 210, close to janitor's room)
Opal, a magazine for... (room 208 with metal cutting saw)
The janitor's notes (janitor's room 207)
Note with password (big chained door, non-missable)
A doctor's business card (locked in a room 106)
A soldier's letter (room with a dead soldier)
First newspaper clipping (room 301, G. Whitehead's office on the table in the next room)
A letter conserning the disappearance
Madelaine's earrings (room after ghost chase, before dropping down)
A written appeal (locked in a room 107 !!!GO THERE BEFORE ENTERING RECEPTION!!! )
The detective's first note (Guilman's room, story-progress item)

Chapter 8:
Missing man's note (beginning of a cave)
Detective's second note (beginning of a cave)
Fragment of Maria's letter (on a table in the cave with big room full of contraptions and footbridges)
Detective's third note (in next level of a cave (-2))
Page from a Bible (in next level of a cave (-2))
Detective's fourth note (in the last level of a cave (-3))
The second article about the accident (in the last level of a cave (-3))

Chapter 10:
Detective's last note (beginning of a chapter, corpse of detective)
Swamp woman's third note (inside Maria's house)
Swamp woman's second note (inside Maria's house)
Draft of Maria's letter to detective (inside Maria's house)
Swamp woman's first note (Inside Maria's house)
Prayer (inside Maria's house, little room where she is standing for a while)

Chapter 12:
Third newspaper clip (on a further table near window in your father's working room)

This achievement needs all hidden messages discovered from objects marked by magnifying lens icon and notes alike. It should unlock by picking last items in chapter 12.

Chapter 1:
Figurines (in attic)
Sleeping angel (in attic)
Cipher on parchment (in attic/story progress)

Chapter 2:
Sleeping angel (dining room)
Figurines (dining room)
Land pre-emption contract (working table next to the dinning room - blocked by armchair)
My father's wallet (next to dining room blocked by armchair, inside the working table drawer)
Pipe (next to dining room blocked by armchair, in the drawer)
Inn photo (upstairs, end of the hallway, near the window)
Second newspaper clipping (in the shed, next to the door)

Chapter 3:
!!FUSE WITH UNKNOWN DEVICE!! (the thing you pull out from butchered deer, examine the hidden message because the item disappear from your inventory once you put it in the fuse box!!!)
Pipe (after painting session with Emma, go upstairs, turn left, in the room on the table)

Chapter 4:
A broken watch (upstairs on the table in your father's room)
An amulet (inside your father's working room after you fall down through the ceiling)
Wooden horse (right in the next room near the piano)
My father's belt (same room as previous item)

Chapter 5:
Broken Kerosene lamp (in the burned inn ruins, at the stairs after you break the white plank)
A page from the Bible (in the room with a safe after you fell down with it)

Chapter 6:
A doctor's business card (locked in a room 106)
First newspaper clipping (room 301, G. Whitehead's office on the table in the next room)

Chapter 7:
A photo of a car (Inside the spy house)

Chapter 9:
Police report (inside the sherrif's car)
Old police report (inside the sherrif's car)

Chapter 10:
Broken Rosary (on the small table inside Maria's house)

Chapter 12:
Third newspaper clip (on a further table near window in your father's working room)
Dancing Figurines (same room, in the closet next to your father working table)

Housekeeping
Find the Key to the Attic

Where it all begins
Complete Chapter 1

Long Arm of Justice
Defeat John Norton

No noise
Shoot down all anomalies

Cold Bastard
Defeat Walter Gilman

The True Beast
Finish the game




Codebreaker
Solve the Enigma
Description: In chapter 3 after you talk with your boss you go to work on Enigma. From what I tested you have to play the game on Standard (medium) or Beast (hardest) difficulty to obtain this achievement since it requires more steps.

If you play on the easiest difficulty just go into "menu -> options -> game" during or before enigma session and switch game difficulty to Standard/Beast, it loads the game from last checkpoint and takes effect immediately. After getting the achie, you can switch back.

Description2(if you are desperate): The small paper note can be turned over with holding left mouse click. Further solution -> It's the row from code table that have all these 3 parameters: V, 03, ST. You really want to have the solution spoiled? -> It's 17th row on the code table.

Newsflash
Hear every news message on the radio
Description: In Chapter 3 after painting session with Emma, everything in your house gets set in, next to the ringing phone there is a radio which after interaction plays one news message per chapter. So listen to it once in every Adam's chapter (3, 5 and 7; there is just a static in further chapters).

Ghostbuster
Chase away Norton's Ghost 10 times in the woods




Eagle Eye
Find 10 hidden messages

Avid reader
Collect 10 notes as Nicolas

Librarian
Collect 10 notes as Adam

Moth
Light 30 candles

Boy with the matches
Collect 50 boxes of matches

Full tank:
Collect 50 bottles of kerosene

Gonzo journalist
Collect all newspaper clippings
Description: There are 3 newspaper clippings. Chapter 2 has "second newspaper clipping" inside a shed, Chapter 6 has "first newspaper clipping" at the Whitehead's room (nr. 301) and the last one is in chapter 12 on a small table near window at your father's working room.

Agent H.
Find all Harold's notes
Description: 5 letters/notes in chapter 7 in five hidden chests. FIND ALL 5 OF THEM BEFORE FINISHING THE CHAPTER!

Bookworm
Collect all notes as Adam

Erudite
Collect all notes as Nicolas

No secrets
Find all hidden messages
Description: There are notes and items scattered all over the house/locations (statues, smoking pipes, newspaper lists, messages, etc.) which have hidden messages on them when examining.




Breaking with the past
Choice No. 1 (Good Nicolas)

Awakening the Beast
Choice No. 2 (Evil Nicolas)


Closed ward
Ending No. 1_1 (Adam is sick)

Spycraft
Ending No. 1_2 (Adam kills the doctor)

Sparrow
Ending No. 2_1 (Adam kills Emma)

The End of Madness
Ending No. 2_2 (Adam kills himself)




Citizen of Blackstone
Unlock all achievements

In the book on table there is a full page of numbers, the number which is missing is the number for the position of the lock.

Combination: 0a31f654675166456f13a0

Using the map from Tent camp you have to find 5 chests with locks on them. To unlock chests you have to use your decipher disc, correct set of outer and inside ring is written somewhere around the place of hidden chest (it's different for each one), then use back of the map to decipher the password for right chest.

!!!You have to find ALL 5 of them in Chapter 7 before going to next chapter!!!

1st:
Before crossing the bridge with hanged people tree.
Cipher: D=S
Password: AGENCY

2nd:
Inside the cave, chained gate (can be cut with bolt cutters).
Cipher: U=B
Password: DEFEAT

3rd:
After leaving your house, scanning the sillouete going up the hill, going throgh stone collapse using pickaxe, before crossing stone bridge, you turn right.
Cipher: K=T
Password: HIDDEN

4th:
Go further beyond the hut with blue cable switch until you meet sunk cottage.
Cipher: X=C
Password: NATION

5th:
After going through fence, keep on right side of the area till you pass stone statue, go further all up to the graveyard.
Cipher: M=W
Password: SECURE









Map:
(there is supposed to be a picture of map above this text, please let me know if Steam made it disappear once again.)

Depends on the difficulty you have to find combinations of letters and numbers to fill into computer.

In the particular order:
A-x, B-x, C-x, D-x

Spoiler numbers:
A-3,B-4,C-1,D-2

You have to pass the cave tunnels in right order, which is written on detective's notes. It doesn't matter if you go through wrong tunnel once you went into correct ones, game doesn't penalize you for that.

1st cave: VI, I, III
2nd cave: IX, VII, VIII
3rd cave: XVII, XIII, XIV

From the comments it should be possible to repeat the endings if force closed the game before credits appear. Seems the game saves the ending choices properly even with ALTF4ing, cheers.

By Siege429:

1. Pick an ending.
2. Pick a secondary ending.
3. Exit out before credits.
4. Start back up and continue.

Location of save files is in:

%localAppData%\TheBeastInside\Saved\SaveGames
or
AppData\Local\TheBeastInside\Saved\SaveGames

!!! DO NOT TOUCH AchievementsGlobal.sav !!! as it is cummulative file that stores every and each achievement you gathered while playing. Rewriting this file with older one may make you miss achievements you already gathered in the game, therefore not unlocking "Citizen of Blackstone" (Unlock all achievements).


Achievements.sav is the file that stores gained achievements in the current playthrough, gets deleted once you finish the game.

Both files are readable with Notepad++, not fit to edit but you can get the basic info of your progress in it.

It is advicable to backup your save files (except AchievementsGlobal.sav !) in chapter 13 so you won't need to play entire game to see other endings too.


THIS GAME HAS NO CHAPTER SELECTION AFTER FINISHING IT. (at the time of writing this guide)