Here is the full list of all 23 What the Fog achievements.
Complete a run on any difficulty.
Complete a run on "Normal".
Complete a run on "Hard".
Complete a run on "Very Hard".
Complete a run on "Spooked".
Complete a run on "Panicked".
Complete a run on "Terrified".
Complete a run with Feng Min.
Complete a run with Claudette Morel.
Complete a run with Dwight Fairfield.
Revive ally 5 times.
Die for the first time.
Die 3 times in the same level.
Pick-up 100 items.
Survive 100 levels.
Defeat the Mega Swarmer.
Defeat the Belcher Boss.
Defeat the Speed Dicer.
Defeat the Plated Bull Slug.
Defeat the Spikespitter Overlord.
Defeat the Bombfield Erupter.
Defeat the Illusionist Surveyor.
Defeat the Knocker Moth Matriarch.