Here is the full list of all 20 Unforgotten achievements.
Receive a gift from the mysterious girl.
Win a fight with the man with deer skin.
Finish the game as the cult leader.
Solve a puzzle in the mine and obtain Egor's knife.
Seduce a girl in a dark cloak.
Complete the prologue to wake up in a strange house.
Peek at the old shaman.
Find a mysterious altar in the forest and rest near it.
Obtain an amulet fighting with Egor.
Start a new day in the weird settlement.
Get a potion from a chest.
Break a chest in an attempt to open it.
Find an old photograph in the kitchen.
Quarrel with Egor to get imprisoned.
Persuade the man with deer skin not to fight to the death.
Craft a stone knife while being held captive.
Persuade the villagers to free the man with deer skin.
Knife Egor.
Finish the game becoming a ritual sacrifice.
Finish the game escaping from the weird settlement.