Here is the full list of all 26 Inside the Backrooms achievements.
You are inside the backrooms
You has been killed by the entity #8 of the backrooms.
You has been killed by the entity #3 of the backrooms.
You has been killed by the unknown entity of the backrooms.
You escaped from the dark rooms
You escaped from the office rooms
You has been killed by the entity #67 of the backrooms.
You has been killed by the entity #10 of the backrooms.
You escaped from the backrooms.
You has been killed by the entity #5 of the backrooms.
Escape from the backrooms without dying. (Starting from Level 0)
Complete the entire game in less than 40 minutes. (Starting from level 0)
Complete the game in the "easy" difficulty. (Starting from the lobby and going to the main ending)
Complete the game in the "Normal" difficulty. (Starting from the lobby and going to the main ending)
Complete the game in the hardest difficult. (Starting from the lobby and going to the main ending)
Complete the Dark Rooms with at least 3 entities mods enabled.
Complete the Office Level with at least 1 entity mod enabled
Complete the sewers levle with at least 3 entities mods enabled
Complete the Terror Hotel level with at least 3 entities mods enabled
Complete the grass corridors level with at least 5 entities mods enabled