Here is the full list of all 20 Unforgotten achievements.
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.
Knife Egor.
Finish the game becoming a ritual sacrifice.
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.
Receive a gift from the mysterious girl.
Win a fight with the man with deer skin.
Finish the game escaping from the weird settlement.
Complete the prologue to wake up in a strange house.
Break a chest in an attempt to open it.
Find an old photograph in the kitchen.
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.
Persuade the villagers to free the man with deer skin.