Decay Part 1 Achievements Guide

Full guide to all the new achievements added, with screenshots. Very very very major spoilers, of course.

Shots & Such Route

There are two main routes, Shots and Such and Cliffhanger.
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The achievements I care about (Shots and Such route):

Prerequistes from Episode 2 & 3 for this route:

- Have gun ammo (Shoot the Hitman once)
- Don't trust Andrew with the parents (duh, this is for Decay to happen in the first place)
- Choose to shoot Andrew in Ashley's vision at the end of Episode 2
- Obtain the Engagement Ring from the woods next to the gas station
- Say you are Andrew in the car

You can obtain the ring by taking the upper path in this section of the woods. It's possible to lock yourself out of this puzzle, so thread wisely. Or just save like a reasonable person.
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Now that you're on the route, there's three achievements to nab here:
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Prophecy Fufiller

After sacrificing the Camper family and walking through the Entity's Portal, fufill Ashley's vision, chase her down and kill her once she reaches a dead end.
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Shots and Such

Instead of catching Ashley in the dead end, wait around and do nothing until the Entity shows up to save her sorry ass. Progress through the game and watch thrity to sixty minutes of wifebeating until you get near the end in the apartment where Andrew notices the engagement ring. For Shots and Such ending, pick up the engagement ring.
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Splat!

For the Deadest of Dead Ends ending, choose not to pick up the ring.

Cliffhanger Route

I think the only prerequistite for this route is to have an empty gun so you actually have to use your words this time instead of just beating the ♥♥♥♥ out of each other. Alternatively, you can have bullets but choose not to kill Andrew in Ashley's vision in Episode 2 and still get this route.

Full Marks!

Three missable achievements in the section with Something Terrifying (Lord Unknown). First of which is Andy's quiz in the first room with documents. After filing the Document on the ground the phone will ring. Answer all three questions correctly. Answer is FALSE, FALSE, FALSE.
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Little Mathematician

Pretty hard to miss this one, it's in the second door of the section. Talk to LeyLey then interact with the papers on the couch, and solve the ever-so-difficult preschooler math.
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Sister Slaughterer

In the third (Theatre) door, go to the back room and kill every single Ashley there. There's a lot of dead Ashley's in this Episode.
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Decaying Along

Self explanatory description. Survive Episode 3: Decay. Finish the section with Lord Unknown/Something Terrifying and progress to Episode 4. (TBD)

Other Achievements

Anything else you missed can be found here.
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Time Capsule

At the flashback at your Grandparents' house at the start of Decay, progress until you are asked to pick blueberries, move to the outer yard and dig up Mom and Dad's capsule next to this bush. You need the Trowel (Shovel) from the garden to dig it up.
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Undetected

At the Gas Station after browsing magazines and the cop goes to grab a sandwich, there is a brief moment where you regain WASD control of Andrew. Walk him to the right to avoid bumping into the cop, pick up Ashley from the bathroom, return the key without running into either cop, and get the hell out.
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----- In a Box

In the argument with Ashley in the car after leaving the Gas Station, concede and accept becoming Andy. Progress the game like normal with Ashley in charge until you reach the LeyLey wins ending, rotting away for eternity safe and sound!
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No Witnesses

After brutally sacrificing two campers purely for your personal gain, chase their kid down until he locks himself in their car. Grab this rock, bash the window in, corner the kid and slit his throat! You killed another child! Congratulations! Mission complete!
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Happy End!

When the Entity/Demon/??? offers you a trip to his realm, wait around for five seconds for the cops to show up and pump you full of lead. Yay!
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A bit more...

A single Episode 1 achievement. Vigorating.

Warden Wins!

In Episode 1, after eating your neighbour cultist and sleeping the warden will come check on room 405. After cleaning up the scene, attempt to escape the warden by leaving to the balcony instead of hiding in the closet.

Old Achievements

Guide for the 4 missable achievements that were in the previous patch:
https://steamcommunity.com/sharedfiles/filedetails/?id=3050231198
(Hitman Wins!, Vision Watcher, Unknown Summon, Present)
EDIT: Looks like the creator of this guide is updating it to include Episode 3 achievements, making this guide ♥♥♥♥♥♥♥ irrelevant! Yippee!

The remaining two achievements are just for clearing Episodes 1 & 2.

[Banake]

Thanks. \o

[Ismael144p]

TYY BRO YOU SAVED ME

[sangetsuki14122]

this def help a lot thanks

[Golira]

Mfw someone uploaded a guide a few hours before