Amnesia: The Dark Descent

Story-forced. You mix a jar of acid from ingredients you gather, then pour it on the organic Shadow-flesh barrier blocking the way forward near the wine cellar/refinery. Required to progress, so it unlocks naturally.

In the Justine DLC, save all three trapped victims (Alois, Basile and Malo) in one playthrough without any of them dying. Mutually exclusive with Egotist and Vacillator within a single run, so it needs its own playthrough.

The "good" ending, and one of the three mutually-exclusive finales (keep a pre-finale save). Earlier in the game you must brew Weyer's Tonic and use it on Agrippa to take his still-living head; carry it all the way to the finale and use it on the portal when it opens during Alexander's monologue. Mutually exclusive with Survivor and Sacrifice.

The scripted elevator crash - the lift you ride breaks and plunges you down to the Prison. It's on the main path, so it can't be missed.

In Justine, let all three victims die during one playthrough. Mutually exclusive with Altruist and Vacillator - needs its own run.

Survive the flooded Cellar Archives - cross via crates and boxes to stay out of the water, distract the invisible water-lurker (the Kaernk) with thrown chunks of meat, and reach the exit. Required to move on.

Unlock all 17 other achievements and this one pops automatically - the last tick.

Complete the main game on Hard Mode (added in a later patch). Hard Mode turns off manual saving, drains sanity and health faster and makes the monsters deadlier - effectively a careful full replay. Best done once you know the game.

Pick up and read every note - documents and diary entries - across BOTH the main game and the Justine DLC. Some are easy to walk past, so keep a note checklist per level.

A joke one. Right at the start, after Daniel wakes in the corridor, just quit to the main menu instead of carrying on. That's it.

In the Cistern, work the mechanism to drain the water and open the route onward to the sewers. Main-path progression, so unmissable.

A deliberate-death achievement. After you break out of your cell with the Shadow (the collapsing red corruption) chasing you, stop or turn back into it and let it consume you rather than escaping. Save first if you don't want to lose progress.

Gather the scattered orb fragments over the course of the game and rebuild the Orb on its stand near the end. Required to reach the finale.

One of the three endings (keep a pre-finale save). In the Orb Chamber, do nothing - don't touch the pillars and don't use Agrippa's head. Wait it out; Alexander completes the ritual and escapes, and the Shadow takes Daniel. Mutually exclusive with Survivor and Benefactor.

In the Justine DLC, find and read the specific hidden note about the "Span-space Device", tucked away behind a book on a shelf in the library area.

One of the three mutually-exclusive endings - keep a save right before the finale so you can get all three. In the Orb Chamber, knock over all three pillars before the portal fully opens; this wrecks the ritual, the Shadow drags Alexander away, and Daniel survives. Mutually exclusive with Sacrifice and Benefactor.

In one Justine playthrough, save at least one victim while letting at least one other die. Mutually exclusive with Altruist and Egotist.